
EXPLAIN ANALYZE
===============

Synopsis
--------

``` sql
EXPLAIN ANALYZE [VERBOSE] statement
```

Description
-----------

Execute the statement and show the distributed execution plan of the statement along with the cost of each operation.

The `VERBOSE` option will give more detailed information and low-level statistics; understanding these may require knowledge of openLooKeng internals and implementation details.


**Note**

*The stats may not be entirely accurate, especially for queries that* *complete quickly.*

Examples
--------

In the example below, you can see the CPU time spent in each stage, as well as the relative cost of each plan node in the stage. Note that the relative cost of the plan nodes is based on wall time, which may or may not be correlated to CPU time. For each plan node you can see some additional statistics (e.g: average input per node instance, average number of hash collisions for relevant plan nodes). Such statistics are
useful when one wants to detect data anomalies for a query (skewness, abnormal hash collisions).

``` sql
lk:sf1> EXPLAIN ANALYZE SELECT count(*), clerk FROM orders WHERE orderdate > date '1995-01-01' GROUP BY clerk;

                                          Query Plan
-----------------------------------------------------------------------------------------------
Fragment 1 [HASH]
    Cost: CPU 88.57ms, Input: 4000 rows (148.44kB), Output: 1000 rows (28.32kB)
    Output layout: [count, clerk]
    Output partitioning: SINGLE []
    - Project[] => [count:bigint, clerk:varchar(15)]
            Cost: 26.24%, Input: 1000 rows (37.11kB), Output: 1000 rows (28.32kB), Filtered: 0.00%
            Input avg.: 62.50 lines, Input std.dev.: 14.77%
        - Aggregate(FINAL)[clerk][$hashvalue] => [clerk:varchar(15), $hashvalue:bigint, count:bigint]
                Cost: 16.83%, Output: 1000 rows (37.11kB)
                Input avg.: 250.00 lines, Input std.dev.: 14.77%
                count := "count"("count_8")
            - LocalExchange[HASH][$hashvalue] ("clerk") => clerk:varchar(15), count_8:bigint, $hashvalue:bigint
                    Cost: 47.28%, Output: 4000 rows (148.44kB)
                    Input avg.: 4000.00 lines, Input std.dev.: 0.00%
                - RemoteSource[2] => [clerk:varchar(15), count_8:bigint, $hashvalue_9:bigint]
                        Cost: 9.65%, Output: 4000 rows (148.44kB)
                        Input avg.: 4000.00 lines, Input std.dev.: 0.00%

Fragment 2 [tpch:orders:1500000]
    Cost: CPU 14.00s, Input: 818058 rows (22.62MB), Output: 4000 rows (148.44kB)
    Output layout: [clerk, count_8, $hashvalue_10]
    Output partitioning: HASH [clerk][$hashvalue_10]
    - Aggregate(PARTIAL)[clerk][$hashvalue_10] => [clerk:varchar(15), $hashvalue_10:bigint, count_8:bigint]
            Cost: 4.47%, Output: 4000 rows (148.44kB)
            Input avg.: 204514.50 lines, Input std.dev.: 0.05%
            Collisions avg.: 5701.28 (17569.93% est.), Collisions std.dev.: 1.12%
            count_8 := "count"(*)
        - ScanFilterProject[table = tpch:tpch:orders:sf1.0, originalConstraint = ("orderdate" > "$literal$date"(BIGINT '9131')), filterPredicate = ("orderdate" > "$literal$date"(BIGINT '9131'))] => [cler
                Cost: 95.53%, Input: 1500000 rows (0B), Output: 818058 rows (22.62MB), Filtered: 45.46%
                Input avg.: 375000.00 lines, Input std.dev.: 0.00%
                $hashvalue_10 := "combine_hash"(BIGINT '0', COALESCE("$operator$hash_code"("clerk"), 0))
                orderdate := tpch:orderdate
                clerk := tpch:clerk
```

When the `VERBOSE` option is used, some operators may report additional information. For example, the window function operator will output the following:

``` sql
EXPLAIN ANALYZE VERBOSE SELECT count(clerk) OVER() FROM orders WHERE orderdate > date '1995-01-01';

                                          Query Plan
-----------------------------------------------------------------------------------------------
  ...
         - Window[] => [clerk:varchar(15), count:bigint]
                 Cost: {rows: ?, bytes: ?}
                 CPU fraction: 75.93%, Output: 8130 rows (230.24kB)
                 Input avg.: 8130.00 lines, Input std.dev.: 0.00%
                 Active Drivers: [ 1 / 1 ]
                 Index size: std.dev.: 0.00 bytes , 0.00 rows
                 Index count per driver: std.dev.: 0.00
                 Rows per driver: std.dev.: 0.00
                 Size of partition: std.dev.: 0.00
                 count := count("clerk")
 ...
```

See Also
--------

[EXPLAIN](./explain.md)
