
# Properties Reference

This section describes the most important config properties that may be used to tune openLooKeng or alter its behavior when required.


## General Properties

### `join-distribution-type`

> -   **Type:** `string`
> -   **Allowed values:** `AUTOMATIC`, `PARTITIONED`, `BROADCAST`
> -   **Default value:** `AUTOMATIC`
>
> The type of distributed join to use.  When set to `PARTITIONED`, openLooKeng will use hash distributed joins.  When set to `BROADCAST`, it will broadcast the right table to all nodes in the cluster that have data from the left table. Partitioned joins require redistributing both tables using a hash of the join key. This can be slower (sometimes substantially) than broadcast joins, but allows much larger joins. In particular broadcast joins will be faster if the right table is much smaller than the left.  However, broadcast joins require that the tables on the right side of the join after filtering fit in memory on each node, whereas distributed joins only need to fit in distributed memory across all nodes. When set to `AUTOMATIC`, openLooKeng will make a cost based decision as to which distribution type is optimal. It will also consider switching the left and right inputs to the join.  In `AUTOMATIC` mode, openLooKeng will default to hash distributed joins if no cost could be computed, such as if the tables do not have statistics. This can also be specified on a per-query basis using the `join_distribution_type` session property.

### `redistribute-writes`

> -   **Type:** `boolean`
> -   **Default value:** `true`
>
> This property enables redistribution of data before writing. This can eliminate the performance impact of data skew when writing by hashing it across nodes in the cluster. It can be disabled when it is known that the output data set is not skewed in order to avoid the overhead of hashing and redistributing all the data across the network. This can also be specified on a per-query basis using the `redistribute_writes` session property.

### `stack-trace-visible`

> - **Type：** `boolean`
> - **Allowed values：** `true`, `false`
> - **Default value：** `false`
>
> This property make exception stack trace which happen in openLooKeng visible or invisible. While it is set to be `true`, the stack trace is visible for all users. While it is set as default or `false`, the stack trace is invisible for all users.

### `openlookeng.admins`

> - **Type：** `string`
> - **Default value：** `No set`
>
> This property is used to set the admin user. The admin user has the authority to obtain all users query history and download all users WEB UI query results. The admin user is not set by default. When multiple admin users need to be set, use a comma to separate the multiple users.

## http security headers properties

### `http-header.content-security-policy`

> - **Type:** `string`
> - **Default value:** `object-src 'none'`
>
> Property to set the http security header `content-security-policy` .

### `http-header.referrer-policy`

> - **Type:** `string`
> - **Default value:** `strict-origin-when-cross-origin`
>
> Property to set the http security header  `referrer-policy`.

### `http-header.x-content-type-options`

> - **类型:** `string`
> - **Default value:** `nosniff`
>
> Property to set the http security header `content-security-policy` .

### `http-header.x-frame-options`

> - **Type:** `string`
> - **Default value:** `deny`
>
> Property to set the http security header  `content-security-policy`.

### `http-header.x-permitted-cross-domain-policies`

> - **Type=:** `string`
> - **Default value:** `master-only`
>
> Property to set the http security header `x-permitted-cross-domain-policies` .

### `http-header.x-xss-protection`

> - **Type:** `string`
> - **Default value:** `1; mode=block`
>
> Property to set the http security header `http-header.x-xss-protection`.


## Memory Management Properties

### `query.max-memory-per-node`

> -   **Type:** `data size`
> -   **Default value:** `JVM max memory * 0.1`
>
> This is the max amount of user memory a query can use on a worker. User memory is allocated during execution for things that are directly attributable to or controllable by a user query. For example, memory used by the hash tables built during execution, memory used during sorting, etc. When the user memory allocation of a query on any worker hits this limit it will be killed.

### `query.max-total-memory-per-node`

> -   **Type:** `data size`
> -   **Default value:** `JVM max memory * 0.3`
>
> This is the max amount of user and system memory a query can use on a worker. System memory is allocated during execution for things that are not directly attributable to or controllable by a user query. For example, memory allocated by the readers, writers, network buffers, etc. When the sum of the user and system memory allocated by a query on any worker hits this limit it will be killed. The value of `query.max-total-memory-per-node` must be greater than `query.max-memory-per-node`.

### `query.max-memory`

> -   **Type:** `data size`
> -   **Default value:** `20GB`
>
> This is the max amount of user memory a query can use across the entire cluster. User memory is allocated during execution for things that are directly attributable to or controllable by a user query. For example, memory used by the hash tables built during execution, memory used during sorting, etc. When the user memory allocation of a query across all workers hits this limit it will be killed.

### `query.max-total-memory`

> -   **Type:** `data size`
> -   **Default value:** `query.max-memory * 2`
>
> This is the max amount of user and system memory a query can use across the entire cluster. System memory is allocated during execution for things that are not directly attributable to or controllable by a user query. For example, memory allocated by the readers, writers, network buffers, etc. When the sum of the user and system memory allocated by a query across all workers hits this limit it will be killed. The value of `query.max-total-memory` must be greater than `query.max-memory`.

### `memory.heap-headroom-per-node`

> -   **Type:** `data size`
> -   **Default value:** `JVM max memory * 0.3`
>
> This is the amount of memory set aside as headroom/buffer in the JVM heap for allocations that are not tracked by openLooKeng.

### `query.suspend-query-enabled`

> -   **Type:** `boolean`
> -   **Default value:** `false`
>
> Enables running query temporary suspension when system is in low resource situation.

### `query.max-suspended-queries`

> -   **Type:** `integer`
> -   **Default value:** `10`
>
> Maximum number of queries to attempt suspension before starting of killing the queries. This property comes in effect only if `query.suspend-query-enabled` is configured `true`

## Spilling Properties

### `experimental.spill-enabled`

> -   **Type:** `boolean`
> -   **Default value:** `false`
>
> Try spilling memory to disk to avoid exceeding memory limits for the query.
>
>
>
> Spilling works by offloading memory to disk. This process can allow a query with a large memory footprint to pass at the cost of slower execution times. Spilling is supported for aggregations, joins (inner and outer), sorting, and window functions. This property will not reduce memory usage required for other join types.
>
>
>
>Be aware that this is an experimental feature and should be used with care.
>
>
>
> This config property can be overridden by the `spill_enabled` session property.

### `experimental.spill-order-by`

> -   **Type:** `boolean`
> -   **Default value:** `true`
>
> Try spilling memory to disk to avoid exceeding memory limits for the query when running sorting operators. This property must be used in conjunction with the `experimental.spill-enabled` property.
>
>
>
> This config property can be overridden by the `spill_order_by` session property.

### `experimental.spill-window-operator`

> -   **Type:** `boolean`
> -   **Default value:** `true`
>
> Try spilling memory to disk to avoid exceeding memory limits for the query when running window operators; This property must be used in conjunction with the `experimental.spill-enabled` property.
>
>
>
> This config property can be overridden by the `spill_window_operator` session property.


### `experimental.spill-build-for-outer-join-enabled`

> -   **Type:** `boolean`
> -   **Default value:** `false`
>
> Enables spill feature for right-outer and full-outer join operations.
>
>
>
> This config property can be overridden by the `spill_build_for_outer_join_enabled` session property.

### `experimental.inner-join-spill-filter-enabled`

> -   **Type:** `boolean`
> -   **Default value:** `false`
>
> Enables bloom filter based build-side spill matching for probe side spill decision.
>
>
>
> This config property can be overridden by the `inner_join_spill_filter_enabled` session property.


### `experimental.spill-reuse-tablescan`

> -   **Type:** `boolean`
> -   **Default value:** `false`
>
> Try spilling memory to disk to avoid exceeding memory limits for the query when running Reuse Exchange; This property must be used in conjunction with the `experimental.spill-enabled` property.
>
>
>
> This config property can be overridden by the `spill_reuse_tablescan` session property.

### `experimental.spiller-spill-path`

> -   **Type:** `string`
> -   **No default value.** Must be set when spilling is enabled
>
> Directory where spilled content will be written. It can be a comma separated list to spill simultaneously to multiple directories, which helps to utilize multiple drives installed in the system.
>
> When `experimental.spiller-spill-to-hdfs` is to `true`, `experimental.spiller-spill-path` must contain only  a single directory.
>
> It is not recommended to spill to system drives. Most importantly, do not spill to the drive on which the JVM logs are written, as disk overutilization might cause JVM to pause for lengthy periods, causing queries to fail.

### `experimental.spiller-max-used-space-threshold`

> -   **Type:** `double`
> -   **Default value:** `0.9`
>
> If disk space usage ratio of a given spill path is above this threshold, this spill path will not be eligible for spilling.

### `experimental.spiller-threads`

> -   **Type:** `integer`
> -   **Default value:** `4`
>
> Number of spiller threads. Increase this value if the default is not able to saturate the underlying spilling device (for example, when using RAID).

### `experimental.max-spill-per-node`

> -   **Type:** `data size`
> -   **Default value:** `100 GB`
>
> Max spill space to be used by all queries on a single node.

### `experimental.query-max-spill-per-node`

> -   **Type:** `data size`
> -   **Default value:** `100 GB`
>
> Max spill space to be used by a single query on a single node.

### `experimental.aggregation-operator-unspill-memory-limit`

> -   **Type:** `data size`
> -   **Default value:** `4 MB`
>
> Limit for memory used for unspilling a single aggregation operator
> instance.

### `experimental.spill-threshold-reuse-tablescan`

> -   **Type:** `int`
> -   **Default value:** `10 (in MB)`
>
> Limit for memory used for caching pages in Reuse Exchange.

### `experimental.spill-compression-enabled`

> -   **Type:** `boolean`
> -   **Default value:** `false`
>
> Enables data compression for pages spilled to disk

### `experimental.spill-encryption-enabled`

> -   **Type:** `boolean`
> -   **Default value:** `false`
>
> Enables using a randomly generated secret key (per spill file) to encrypt and decrypt data spilled to disk

### `experimental.spill-direct-serde-enabled`

> -   **Type:** `boolean`
> -   **Default value:** `false`
>
> Enables to serialize/read the page directly to/from the stream.

### `experimental.spill-prefetch-read-pages`

> -   **Type:** `integer`
> -   **Default value:** `1`
>
> Sets number of pages prefetched while reading from spilled files.


### `experimental.spill-use-kryo-serialization`

> -   **Type:** `boolean`
> -   **Default value:** `false`
>
> Enables Kryo based serialization for spill to disk, instead of default java serializer.


### `experimental.revocable-memory-selection-threshold`

> -   **Type:** `data size`
> -   **Default value:** `512 MB`
>
> Sets memory selection threshold for revocable memory of operator to directly allocate revocable memory for remaining bytes ready to revoke.

### `experimental.prioritize-larger-spilts-memory-revoke`

> -   **Type:** `boolean`
> -   **Default value:** `true`
>
> Enables to prioritize splits with larger revocable memory.

### `experimental.spill-non-blocking-orderby`

> -   **Type:** `boolean`
> -   **Default value:** `false`
>
> Enables order by operator to use asynchronous mechanism to spill, i.e it can accumulate input even when a spill is in progress and initiate a secondary spill when the secondary data accumulate exceeds a threshold or when the primary spill is completed, the default value of the threshold is the minimum between 20MB and 5% of available free memory. This property must be used in conjunction with the `experimental.spill-enabled` property.
>
>
>
> This config property can be overridden by the `spill_non_blocking_orderby` session property.

### `experimental.spiller-spill-to-hdfs`

> -   **Type:** `boolean`
> -   **Default value:** `false`
>
> Enables spilling into HDFS. When this property is set to `true` the property `experimental.spiller-spill-profile` must be set and also `experimental.spiller-spill-path` must contain only a single path.

### `experimental.spiller-spill-profile`

> -   **Type:** `string`
> -   **No default value.** Must be set when spilling to hdfs is enabled
>
>
> This property defines the [filesystem](../develop/filesystem.md) profile used to spill. The corresponding profile must exist in `etc/filesystem`. For example, if this property is set as `experimental.spiller-spill-profile=spill-hdfs`, a profile describing this filesystem `spill-hdfs.properties` must be created in `etc/filesystem` with necessary information including authentication type, config, and keytabs (if applicable, refer [filesystem](../develop/filesystem.md) for details).
>
> This property is required when `experimental.spiller-spill-to-hdfs` is set to `true`. It must be included in configuration files for all coordinators and all workers. The specified file system must be accessible by all workers, and they must be able to read from and write to the path declared in `experimental.spiller-spill-path` folder in the specified file system.

## Exchange Properties

Exchanges transfer data between openLooKeng nodes for different stages of a query. Adjusting these properties may help to resolve inter-node communication issues or improve network utilization.

### `exchange.client-threads`

> -   **Type:** `integer`
> -   **Minimum value:** `1`
> -   **Default value:** `25`
>
> Number of threads used by exchange clients to fetch data from other openLooKeng nodes. A higher value can improve performance for large clusters or clusters with very high concurrency, but excessively high values may cause a drop in performance due to context switches and additional memory usage.

### `exchange.concurrent-request-multiplier`

> -   **Type:** `integer`
> -   **Minimum value:** `1`
> -   **Default value:** `3`
>
> Multiplier determining the number of concurrent requests relative to available buffer memory. The maximum number of requests is determined using a heuristic of the number of clients that can fit into available buffer space based on average buffer usage per request times this multiplier. For example, with an `exchange.max-buffer-size` of `32 MB` and `20 MB` already used and average size per request being `2MB`, the maximum number of clients is `multiplier * ((32MB - 20MB) / 2MB) = multiplier * 6`. Tuning this value adjusts the heuristic, which may increase concurrency and improve network utilization.

### `exchange.max-buffer-size`

> -   **Type:** `data size`
> -   **Default value:** `32MB`
>
> Size of buffer in the exchange client that holds data fetched from other nodes before it is processed. A larger buffer can increase network throughput for larger clusters and thus decrease query processing time, but will reduce the amount of memory available for other usages.

### `exchange.max-response-size`

> -   **Type:** `data size`
> -   **Minimum value:** `1MB`
> -   **Default value:** `16MB`
>
> Maximum size of a response returned from an exchange request. The response will be placed in the exchange client buffer which is shared across all concurrent requests for the exchange.
>
> Increasing the value may improve network throughput if there is high latency. Decreasing the value may improve query performance for large clusters as it reduces skew due to the exchange client buffer holding responses for more tasks (rather than hold more data from fewer tasks).

### `sink.max-buffer-size`

> -   **Type:** `data size`
> -   **Default value:** `32MB`
>
> Output buffer size for task data that is waiting to be pulled by upstream tasks. If the task output is hash partitioned, then the buffer will be shared across all of the partitioned consumers. Increasing this value may improve network throughput for data transferred between stages if the network has high latency or if there are many nodes in the cluster.

## Failure Recovery handling Properties

### Failure Retry Policies

### `failure.recovery.retry.profile`

> -   **Type:** `String`
> -   **Default value:** `default`
>
> This property defines the failure detection profile used to determine if failure has happened for a http client. The value `<profile-name>` set for this property has to correspond to `<profile-name>.properties` file in `etc/failure-retry-policy/`. In case no such profile is available, and this property is not set, "default" profile is used.
> For example, `failure.recovery.retry.profile="test"` requires `test.properties` file to be present in `etc/failure-retry-policy`.
> The file `test.properties` must contain `failure.recovery.retry.type` specified.


### `failure.recovery.retry.type`

> -   **Type:** `String`
> -   **Default value:** `timeout`
>
> The failure detection mechanism in use. Default is timeout based failure detection.
>
#### `timeout` based failure detection. 
> Using this mechanism, HTTP client failures are retried for a specific duration before considering it as a permanent failure.
> Additional properties `max.error.duration` can be defined for this type of failure detection.
>
#### `max-retry` based failure detection. 
> Using this mechanism, HTTP client failures are retried for a specific number of times before considering it as a permanent failure.
> Additional properties `max.retry.count` and `max.error.duration` can be defined for this type of failure detection. 
> Using this type of failure detection is configured to be used, `max.retry.count` times retry is performed before consulting the failure detector module. When the remote node is failed as per the failure detector module, HTTP client considers it a permanent failure. Otherwise, i.e. When remote worker node is alive but not sending response, retry happens for `max.error.duration` before considering it as permanent failure.

### `max.error.duration`
> -   **Type:** `duration`
> -   **Default value:** `300s`
>
> The maximum amount of time coordinator waits for inter-task related errors to be resolved before it's considered a permanent failure.


### `max.retry.count`

> -   **Type:** `integer`
> -   **Default value:** `100`
>
> The maximum number of retry for failed task performed by the coordinator before consulting the failure detector module about the remote node status. 
> This parameter is the minimum count before consulting the failure detection module. Hence, the actual number of failures may vary slightly based on the cluster size, and load on the cluster. 
> This property is used only for `max-retry` based failure detection profiles. 
> The minimum value for this parameter is 100.

### Gossip Protocol Configurations for Failure Detection

### `failure-detection-protocol`

>- **Type:** String
>- **Default value:** `heartbeat`
>
> This property defines the type of failure detector in use. Default configuration is `heartbeat` failure detector.
> Gossip protocol can be enabled by specifying this parameter in `config.properties` file, with the value `gossip`.
> All nodes (i.e. coordinator as well as workers) in a cluster should have this property specified in their respective `etc/config.properties` file.

### `failure-detector.heartbeat-interval`

>- **Type:** Duration
>- **Default value:** `500ms` (500 miliseconds)
>
> This is the interval of gossip between two nodes in the cluster.
> In gossip protocol, two workers are expected to gossip with higher frequency than the coordinator and a worker.
> In `config.properties` for the coordinator, this property can be set with a reasonably higher value, such as `5s` (5 seconds).
> In workers, this property can be left to use the default value.
>
### `failure-detector.worker-gossip-probe-interval`
>
> - **Type:** Duration
>- **Default value:** `5s` (5 seconds)
>
> Gossip protocol uses monitoring tasks (same as the heartbeat failure detector) to keep tab on the other nodes.
> This property specifies the interval of refreshing the monitoring tasks to trigger worker to worker gossip.
> This property, if needed to be configured with any other value than the default, should be specified only for the worker nodes.
> This parameter should have higher value than `failure-detector.heartbeat-interval`.
>
### `failure-detector.coordinator-gossip-probe-interval`
>
> - **Type:** Duration
>- **Default value:** `5s` (5 seconds)
>
> Gossip protocol uses monitoring tasks (same as the heartbeat failure detector) to keep tab on the other nodes.
> This property specifies the interval of refreshing the monitoring tasks to trigger coordinator to worker gossip.
> This property, if needed to be configured with any other value than the default, should be specified only for the coordinator.
> This parameter should have higher value than `failure-detector.heartbeat-interval` and `failure-detector.worker-gossip-probe-interval`.
>
### `failure-detector.coordinator-gossip-collate-interval`
>
> - **Type:** Duration
>- **Default value:** `2s` (2 seconds)
>
> This property specifies the interval in which the coordinator collates all the gossips it obtained from all the workers.
> This property has to be specified only for the coordinator.
> This parameter should have higher value than `failure-detector.heartbeat-interval`.
>
### `failure-detector.gossip-group-size`
>
> - **Type:** Integer
>- **Default value:** `Integer.MAX_VALUE`
>
> A worker should gossip with how many other workers in the cluster, is defined by this parameter.
> Any value higher than the cluster-size (i.e. the number of workers) implies all-to-all gossip.
> To keep the network overhead low, this value should be reasonably low for a big cluster (e.g. 10 for a cluster size of 100).
> On each refresh of the worker-monitoring tasks at the coordinator, the coordinator defines the list of worker URIs of size `failure-detector.gossip-group-size` to trigger worker-to-worker gossip.

## Task Properties

### `task.concurrency`

> -   **Type:** `integer`
> -   **Restrictions:** must be a power of two
> -   **Default value:** `16`
>
> Default local concurrency for parallel operators such as joins and aggregations. This value should be adjusted up or down based on the query concurrency and worker resource utilization. Lower values are better for clusters that run many queries concurrently because the cluster will already be utilized by all the running queries, so adding more concurrency will result in slow downs due to context switching and other overhead. Higher values are better for clusters that only run one or a few queries at a time. This can also be specified on a per-query basis using the `task_concurrency` session property.

### `task.http-response-threads`

> -   **Type:** `integer`
> -   **Minimum value:** `1`
> -   **Default value:** `100`
>
> Maximum number of threads that may be created to handle HTTP responses. Threads are created on demand and are cleaned up when idle, thus there is no overhead to a large value if the number of requests to be handled is small. More threads may be helpful on clusters with a high number of concurrent queries, or on clusters with hundreds or thousands of workers.

### `task.http-timeout-threads`

> -   **Type:** `integer`
> -   **Minimum value:** `1`
> -   **Default value:** `3`
>
> Number of threads used to handle timeouts when generating HTTP responses. This value should be increased if all the threads are frequently in use. This can be monitored via the `io.prestosql.server:name=AsyncHttpExecutionMBean:TimeoutExecutor` JMX object. If `ActiveCount` is always the same as `PoolSize`, increase the number of threads.

### `task.info-update-interval`

> -   **Type:** `duration`
> -   **Minimum value:** `1ms`
> -   **Maximum value:** `10s`
> -   **Default value:** `3s`
>
> Controls staleness of task information, which is used in scheduling. Larger values can reduce coordinator CPU load, but may result in suboptimal split scheduling.

### `task.max-partial-aggregation-memory`

> -   **Type:** `data size`
> -   **Default value:** `16MB`
>
> Maximum size of partial aggregation results for distributed aggregations. Increasing this value can result in less network transfer and lower CPU utilization by allowing more groups to be kept locally before being flushed, at the cost of additional memory usage.

### `task.max-worker-threads`

> -   **Type:** `integer`
> -   **Default value:** `Node CPUs * 2`
>
> Sets the number of threads used by workers to process splits. Increasing this number can improve throughput if worker CPU utilization is low and all the threads are in use, but will cause an increased heap space usage. Setting the value too high may cause a drop in performance due to a context switching. The number of active threads is available via the `RunningSplits` property of the `io.prestosql.execution.executor:name=TaskExecutor.RunningSplits` JXM object.

### `task.min-drivers`

> -   **Type:** `integer`
> -   **Default value:** `task.max-worker-threads * 2`
>
> The target number of running leaf splits on a worker. This is a minimum value because each leaf task is guaranteed at least `3` running splits. Non-leaf tasks are also guaranteed to run in order to prevent deadlocks. A lower value may improve responsiveness for new tasks, but can result in underutilized resources. A higher value can increase resource utilization, but uses additional memory.

### `task.writer-count`

> -   **Type:** `integer`
> -   **Restrictions:** must be a power of two
> -   **Default value:** `1`
>
> The number of concurrent writer threads per worker per query. Increasing this value may increase write speed, especially when a query is not I/O bound and can take advantage of additional CPU for parallel writes (some connectors can be bottlenecked on CPU when writing due to compression or other factors). Setting this too high may cause the cluster to become overloaded due to excessive resource utilization. This can also be specified on a per-query basis using the `task_writer_count` session property.

## Node Scheduler Properties

### `node-scheduler.max-splits-per-node`

> -   **Type:** `integer`
> -   **Default value:** `100`
>
> The target value for the total number of splits that can be running for each worker node.
>
>
>
> Using a higher value is recommended if queries are submitted in large batches (e.g., running a large group of reports periodically) or for connectors that produce many splits that complete quickly. Increasing this value may improve query latency by ensuring that the workers have enough splits to keep them fully utilized.
>
>
>
>Setting this too high will waste memory and may result in lower performance due to splits not being balanced across workers. Ideally, it should be set such that there is always at least one split waiting to be processed, but not higher.

### `node-scheduler.max-pending-splits-per-task`

> -   **Type:** `integer`
> -   **Default value:** `10`
>
> The number of outstanding splits that can be queued for each worker node for a single stage of a query, even when the node is already at the limit for total number of splits. Allowing a minimum number of splits per stage is required to prevent starvation and deadlocks.
>
>
>
>This value must be smaller than `node-scheduler.max-splits-per-node`, will usually be increased for the same reasons, and has similar drawbacks if set too high.

### `node-scheduler.min-candidates`

> -   **Type:** `integer`
> -   **Minimum value:** `1`
> -   **Default value:** `10`
>
> The minimum number of candidate nodes that will be evaluated by the node scheduler when choosing the target node for a split. Setting this value too low may prevent splits from being properly balanced across all worker nodes. Setting it too high may increase query latency and increase CPU usage on the coordinator.

### `node-scheduler.network-topology`

> -   **Type:** `string`
> -   **Allowed values:** `legacy`, `flat`
> -   **Default value:** `legacy`
>
> Sets the network topology to use when scheduling splits. `legacy` will ignore the topology when scheduling splits. `flat` will try to schedule splits on the host where the data is located by reserving 50% of the work queue for local splits. It is recommended to use `flat` for clusters where distributed storage runs on the same nodes as openLooKeng workers.

## Optimizer Properties

### `optimizer.dictionary-aggregation`

> -   **Type:** `boolean`
> -   **Default value:** `false`
>
> Enables optimization for aggregations on dictionaries. This can also be specified on a per-query basis using the `dictionary_aggregation` session property.

### `optimizer.optimize-hash-generation`

> -   **Type:** `boolean`
> -   **Default value:** `true`
>
> Compute hash codes for distribution, joins, and aggregations early during execution, allowing result to be shared between operations later in the query. This can reduce CPU usage by avoiding computing the same hash multiple times, but at the cost of additional network transfer for the hashes. In most cases it will decrease overall query processing time. This can also be specified on a per-query basis using the `optimize_hash_generation` session property.
>
>
>
> It is often helpful to disable this property when using [EXPLAIN](../sql/explain.md) in order to make the query plan easier to read.

### `optimizer.optimize-metadata-queries`

> -   **Type:** `boolean`
> -   **Default value:** `false`
>
> Enable optimization of some aggregations by using values that are stored as metadata. This allows openLooKeng to execute some simple queries in constant time. Currently, this optimization applies to `max`, `min` and `approx_distinct` of partition keys and other aggregation insensitive to the cardinality of the input (including `DISTINCT` aggregates). Using this may speed up some queries significantly.
>
>
>
> The main drawback is that it can produce incorrect results if the connector returns partition keys for partitions that have no rows. In particular, the Hive connector can return empty partitions if they were created by other systems (openLooKeng cannot create them).

### `optimizer.push-aggregation-through-join`

> -   **Type:** `boolean`
> -   **Default value:** `true`
>
> When an aggregation is above an outer join and all columns from the outer side of the join are in the grouping clause, the aggregation is pushed below the outer join. This optimization is particularly useful for correlated scalar subqueries, which get rewritten to an aggregation over an outer join. For example:
>
>
>
> ```sql
>SELECT * FROM item i
>        WHERE i.i_current_price > (
>            SELECT AVG(j.i_current_price) FROM item j
>                WHERE i.i_category = j.i_category);
> ```
>
>
>
> Enabling this optimization can substantially speed up queries by reducing the amount of data that needs to be processed by the join.  However, it may slow down some queries that have very selective joins. This can also be specified on a per-query basis using the `push_aggregation_through_join` session property.

### `optimizer.push-table-write-through-union`

> -   **Type:** `boolean`
> -   **Default value:** `true`
>
> Parallelize writes when using `UNION ALL` in queries that write data. This improves the speed of writing output tables in `UNION ALL` queries because these writes do not require additional synchronization when collecting results. Enabling this optimization can improve `UNION ALL` speed when write speed is not yet saturated. However, it may slow down queries in an already heavily loaded system. This can also be specified on a per-query basis using the `push_table_write_through_union` session property.

### `optimizer.join-reordering-strategy`

> -   **Type:** `string`
> -   **Allowed values:** `AUTOMATIC`, `ELIMINATE_CROSS_JOINS`, `NONE`
> -   **Default value:** `AUTOMATIC`
>
> The join reordering strategy to use.  `NONE` maintains the order the tables are listed in the query.  `ELIMINATE_CROSS_JOINS` reorders joins to eliminate cross joins where possible and otherwise maintains the original query order. When reordering joins it also strives to maintain the original table order as much as possible. `AUTOMATIC` enumerates possible orders and uses statistics-based cost estimation to determine the least cost order. If stats are not available or if for any reason a cost could not be computed, the `ELIMINATE_CROSS_JOINS` strategy is used. This can also be specified on a per-query basis using the `join_reordering_strategy` session property.

### `optimizer.max-reordered-joins`

> -   **Type:** `integer`
> -   **Default value:** `9`
>
> When optimizer.join-reordering-strategy is set to cost-based, this property determines the maximum number of joins that can be reordered at once.
>
>
>
> **Warning**
>
> The number of possible join orders scales factorially with the number of relations, so increasing this value can cause serious performance issues.

### `hetu.query-pushdown`

> -   **Type:** `boolean`
> -   **Default value:** `true`
>
> Switch for controlling the push-down feature of the JDBC connector and DC connector.

### `optimizer.reuse-table-scan`

> -   **Type:** `boolean`
> -   **Default value:** `false`
>
> Use Reuse Exchange to cache data in memory if the query contains tables or Common Table Expressions(CTE) which are present more than one time with the same projections and filters on them. Enabling this feature will reduce the time taken to execute the query by caching data in memory and avoiding reading from disk multiple times.
> This can also be specified on a per-query basis using the `reuse_table_scan` session property.
>
> Note: when `cte_reuse_enabled` or `optimizer.cte-reuse-enabled` is enabled reuse exchange will be disabled.

### `optimizer.cte-reuse-enabled`

> -   **Type:** `boolean`
> -   **Default value:** `false`
>
> Enable this flag to execute Common Table Expressions (CTE) only once irrespective of number of times same CTE is being used in the main query.
> This will help to improve query execution performance when same CTE is used more than once.
> This can also be specified on a per-query basis using the `cte_reuse_enabled` session property.

### `optimizer.sort-based-aggregation-enabled`

> -   **Type:** `boolean`
> -   **Default value:** `false`
>
> Sort based aggregation is used when underlying source is in pre-sorted order, this is used instead of Hash aggregation which take more footprint to build hash tables.
> Sort based aggregation used less memory foot print when compared to hash aggregation.
> Conditions when Sort based aggregation in case of Hive
> - 1) Grouping columns should be same or less than sorted columns and it should be in the same order.
> - 2) Joins case probe side table should be sorted and join criteria should be same or less than sorted columns and it should be in the same order.
> - 3) bucket_count is 1 bucketed_by columns should be same or less than Grouping columns and it should be in the same order.
> - 4) bucket_count is more than 1 bucketed_by columns should be same as Grouping columns and it should be in the same order.
> - 5) In case of partition table, Grouping columns should contain all partitions in same order following by the subset of sorted by columns.
> - 6) When distinct is used, Grouping columns followed by a distinct column should be subset of sorted by columns.
>
> This can also be specified on a per-query basis using the `sort_based_aggregation_enabled` session property.
>
> **Note:** This is supported only for Hive connector.

## Regular Expression Function Properties

The following properties allow tuning the [regexp](../functions/regexp.md).

### `regex-library`

> -   **Type:** `string`
> -   **Allowed values:** `JONI`, `RE2J`
> -   **Default value:** `JONI`
>
> Which library to use for regular expression functions. `JONI` is generally faster for common usage, but can require exponential time for certain expression patterns. `RE2J` uses a different algorithm which guarantees linear time, but is often slower.

### `re2j.dfa-states-limit`

> -   **Type:** `integer`
> -   **Minimum value:** `2`
> -   **Default value:** `2147483647`
>
> The maximum number of states to use when RE2J builds the fast but potentially memory intensive deterministic finite automaton (DFA) for regular expression matching. If the limit is reached, RE2J will fall back to the algorithm that uses the slower, but less memory intensive non-deterministic finite automaton (NFA). Decreasing this value decreases the maximum memory footprint of a regular expression search at the cost of speed.

### `re2j.dfa-retries`

> -   **Type:** `integer`
> -   **Minimum value:** `0`
> -   **Default value:** `5`
>
> The number of times that RE2J will retry the DFA algorithm when it reaches a states limit before using the slower, but less memory intensive NFA algorithm for all future inputs for that search. If hitting the limit for a given input row is likely to be an outlier, you want to be able to process subsequent rows using the faster DFA algorithm. If you are likely to hit the limit on matches for subsequent rows as well, you want to use the correct algorithm from the beginning so as not to waste time and resources. The more rows you are processing, the larger this value should be.

## Heuristic Index Properties

Heuristic index is external index module that which can be used to filter to out rows at the connector level. Bitmap, Bloom, MinMaxIndex are list of indexes provided by openLooKeng. As of now, bitmap index supports supports hive connector for tables with ORC storage format.

### `hetu.heuristicindex.filter.enabled`

> -   **Type:** `boolean`
> -   **Default value:** `false`
>
> This property enables heuristic index. There is also a session property `heuristicindex_filter_enabled` which can be set per session. Note: the session property should ONLY be used to turn on and off index filtering temporarily when this global property in config file is set as `true`. The session property CANNOT be used to turn on index filter when it's not enabled globally.

### `hetu.heuristicindex.filter.cache.max-memory`

> -   **Type:** `data size`
> -   **Default value:** `10GB`
>
> Caching the index files provides better performance, index files are read only and modified very rarely. Caching saves time spent on reading the files from indexstore. This property controls the maximum memory used by the index cache. When limit exceeded, existing entries will be removed from cache based on LRU and new entry will be added to cache.

### `hetu.heuristicindex.filter.cache.soft-reference`

> -   **Type:** `boolean`
> -   **Default value:** `true`
>
> Caching the index files provides better performance, however it utilizes memory. Enabling this property allows the Garbage Collector to remove entries from the cache if memory is running low.
>
> Be aware that this is an experimental feature and should be used with care.

### `hetu.heuristicindex.filter.cache.ttl`

> -   **Type:** `Duration`
> -   **Default value:** `24h`
>
> The time period after which index cache expires.

### `hetu.heuristicindex.filter.cache.loading-threads`

> -   **Type:** `integer`
> -   **Default value:** `10`
>
> The number of threads used to load indices in parallel.

### `hetu.heuristicindex.filter.cache.loading-delay`

> -   **Type:** `Duration`
> -   **Default value:** `10s`
>
> The delay to wait before async loading task starts to load index cache from indexstore.

### `hetu.heuristicindex.filter.cache.preload-indices`

> - **Type**：`string`
> - **Default value：** ``
>
> Preload the specified indices (comma-separated) when the server starts. Put `ALL` to load all indices.

### `hetu.heuristicindex.indexstore.uri`

> -   **Type:** `string`
> -   **Default value:** `/opt/hetu/indices/`
>
> Directory under which all index files are stored. Each index will be stored in its own subdirectory.

### `hetu.heuristicindex.indexstore.filesystem.profile`

> -   **Type** `string`
>
> This property defines the filesystem profile used to read and write index. The corresponding profile must exist in `etc/filesystem`. For example, if this property is set as `hetu.heuristicindex.filter.indexstore.filesystem.profile=index-hdfs1`, a profile describing this filesystem access `index-hdfs1.properties` must be created in `etc/filesystem` with necessary information including authentication type, config, and keytabs (if applicable).
>
> `LOCAL` filesystem type should only be used during testing or in single node clusters.
>
> `HDFS` filesystem type should be used in production in order for the index to be accessible by all nodes in the cluster. All nodes should be configured to use the same filesystem profile.

## Execution Plan Cache Properties

Execution plan cache feature allows the coordinator to reuse execution plans between identical queries, instead
of constructing another execution plan, thus reducing the amount of query pre-processing required.

### `hetu.executionplan.cache.enabled`
>
> -    **Type:** `boolean`
> -    **Default value:** `false`
>
> Enable or disable execution plan cache. Disabled by default.

### `hetu.executionplan.cache.limit`
>
> -  **Type:** `integer`
> - **Default value:** `10000`
>
> Maximum number of execution plans to keep in the cache

### `hetu.executionplan.cache.timeout`
>
> - **Type:** `integer`
> - **Default value:** `86400000 ms`
>
> Time in milliseconds to expire cached execution plans after the last access

## SplitCacheMap Properties

SplitCacheMap must be enabled to support caching row data. When enabled, the coordinator stores table, partition and split scheduling metadata that
helps with cache affinity scheduling.

### `hetu.split-cache-map.enabled`

> -   **Type:** `boolean`
> -   **Default value:** `false`
>
> This property enables split caching functionality.
> If state store is enabled, the split cache map configuration is automatically replicated in state store as well.
> In case of HA setup with multiple coordinators, the state store is used to share split cache map between the coordinators.

### `hetu.split-cache-map.state-update-interval`

> -   **Type:** `integer`
> -   **Default value:** `2 seconds`
>
> This property controls how frequently the split cache map is updated in state store. It is primarily applicable for HA deployment.

## Auto-Vacuum

> Auto-Vacuum enables the system to automatically manage vacuum jobs by constantly monitoring the tables which needs vacuum in order to maintain optimal performance.
> Engine gets the tables from data sources that are eligible for vacuum and trigger vacuum operation for those tables.

### `auto-vacuum.enabled`

> -   **Type:** `boolean`
> -   **Default value:** `false`
>
> This is to enable auto-vacuum functionality.
>
> **Note:** This should be configured only in coordinator.

### `auto-vacuum.scan.interval`

> -   **Type:** `Duration`
> -   **Default value:** `10m`
>
> It is scheduled Interval to get vacuum table info from the data sources and trigger vacuum for those tables.
> Timer is started when the server starts and it will keep on scheduling for configured interval.
> Min value is 15s and Max value is 24h.
>
> **Note:** This should be configured only in coordinator.

### `auto-vacuum.scan.threads`

> -  **Type:** `integer`
> - **Default value:** `3`
>
> Number of threads used for Auto vacuum functionality. Min value is 1 and Max value is 16.
>
> **Note:** This should be configured only in coordinator.

## CTE (Common Table Expression) Properties

### `cte.cte-max-queue-size`

> -   **Type:** `int`
> -   **Default value:** `1024`
>
> The maximum number of pages per processing queue.  The number of processing queues is equal to the number of the CTE references in the main query.
> This can also be specified on a per-query basis using the `cte_max_queue_size` session property.

### `cte.cte-max-prefetch-queue-size`

> -   **Type:** `int`
> -   **Default value:** `512`
>
> The maximum number of pages which can be held by the prefetch queue when processing queue is full. Prefetch queues are used to eagerly fetch the data so that query execution need not wait on I/O.
> This can also be specified on a per-query basis using the `cte_max_prefetch_queue_size` session property.
>
> **Note:** This should be configured in all workers.

## Sort Base aggregation Properties

### `sort.prcnt-drivers-for-partial-aggr`

> -   **Type:** `int`
> -   **Default value:** `5`
>
> In Sort based aggregation percentage of number of drivers that are used for unfinalized/partial values.
> This can also be specified on a per-query basis using the `prcnt_drivers_for_partial_aggr` session property.
>
> **Note:** This should be configured on all nodes .


## Query Manager

### `query.remote-task.max-error-duration`

> -   **Type:** `duration`
> -   **Default value:** `5m`
>
> The maximum time coordinator waits for remote-task related error to be resolved before it's considered a failure.
>
> Note:
> For snapshot recovery `query.remote-task.max-error-duration` should be greater than `exchange.max-error-duration`.

## Query Recovery

### `recovery_enabled`

> -   **Type:** `boolean`
> -   **Default value:** `false`
>
> This session property is used to enable or disable the recovery framework, which enables to restart/resume the query in case of failure.

### `snapshot_enabled`

> -   **Type:** `boolean`
> -   **Default value:** `false`
>
> This session property is enabled to capture snapshots during query execution, when recovery framework is enabled. Without recovery framework enabled this flag has no significance

### `hetu.experimental.snapshot.profile`

> -   **Type:** `string`
>
> This property defines the [filesystem](../develop/filesystem.md) profile used to stored snapshots. The corresponding profile must exist in `etc/filesystem`. For example, if this property is set as `hetu.experimental.snapshot.profile=snapshot-hdfs1`, a profile describing this filesystem `snapshot-hdfs1.properties` must be created in `etc/filesystem` with necessary information including authentication type, config, and keytabs (if applicable). Please refer to the [filesystem](../develop/filesystem.md) section for details.
>
> This property is required if any query is executed with distributed snapshot turned on. It must be included in configuration files for all coordinators and all workers. The specified file system must be accessible by all workers, and they must be able to read from and write to the `/tmp/hetu/snapshot` folder in the specified file system.
>
> This is an experimental property. In the future it may be allowed to store snapshots in non-file-system locations, e.g. in a connector.

### `hetu.recovery.maxRetries`

> -   **Type:** `int`
> -   **Default value:** `10`
>
> This property defines the maximum number of error recovery attempts for a query. When the limit is reached, the query fails.
>
> This can also be specified on a per-query basis using the `recovery_max_retries` session property.

### `hetu.recovery.retryTimeout`

> -   **Type:** `duration`
> -   **Default value:** `10m` (10 minutes)
>
> This property defines the maximum amount of time for the system to wait until all tasks are successfully restored. If any task is not ready within this timeout, then the recovery attempt is considered a failure, and the query will try to resume from an earlier snapshot if available.
>
> This can also be specified on a per-query basis using the `recovery_retry_timeout` session property.

### `hetu.snapshot.useKryoSerialization`

> -   **Type:** `boolean`
> -   **Default value:** `false`
>
> Enables Kryo based serialization for snapshot, instead of default java serializer.

### `experimental.eliminate-duplicate-spill-files`

> -   **Type:** `boolean`
> -   **Default value:** `false`
>
> Enables elimination of duplicate spill files storage as part of snapshot capture.



## HTTP Client Configurations

### `http.client.idle-timeout`

> -   **Type:** `duration`
> -   **Default value:** `30s` (30 seconds)
>
> This property defines the time for which a given http client shall stay connected without any operations performed over it.
> After the specified time elapse with no activity, then the client connection is closed and related resources are released.
>
> (Note: this parameter should be configured with higher time when in high load environment)

### `http.client.request-timeout`

> -   **Type:** `duration`
> -   **Default value:** `10s` (10 seconds)
>
> This property defines the time threshold for a given http client for which response should be received.
> After the configured time elapsed and no response received, then client connection consider that to be failure in submission of request.
>
> (Note: this parameter should be configured with higher time when in high load environment)

## Connector Properties configuration

### `case-insensitive-name-matching`
>
> -    **Type:** `boolean`
> -    **Default value:** `false`
>
> Case-insensitive matching between database and collection names. The default is case sensitive.
