
EXPLAIN
=======

Synopsis
--------

``` sql
EXPLAIN [ ( option [, ...] ) ] statement

where option can be one of:

    FORMAT { TEXT | GRAPHVIZ | JSON }
    TYPE { LOGICAL | DISTRIBUTED | VALIDATE | IO }
```

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

Show the logical or distributed execution plan of a statement, or validate the statement. Use `TYPE DISTRIBUTED` option to display fragmented plan. Each plan fragment is executed by a single or multiple
openLooKeng nodes. Fragments separation represent the data exchange between openLooKeng nodes. Fragment type specifies how the fragment is executed by openLooKeng nodes and how the data is distributed between fragments:

`SINGLE`

Fragment is executed on a single node.

`HASH`

Fragment is executed on a fixed number of nodes with the input data distributed using a hash function.

`ROUND_ROBIN`

Fragment is executed on a fixed number of nodes with the input data distributed in a round-robin fashion.

`BROADCAST`

Fragment is executed on a fixed number of nodes with the input data broadcasted to all nodes.

`SOURCE`

Fragment is executed on nodes where input splits are accessed.

Examples
--------

Logical plan:

``` sql
lk:tiny> EXPLAIN SELECT regionkey, count(*) FROM nation GROUP BY 1;
                                                Query Plan
----------------------------------------------------------------------------------------------------------
 - Output[regionkey, _col1] => [regionkey:bigint, count:bigint]
         _col1 := count
     - RemoteExchange[GATHER] => regionkey:bigint, count:bigint
         - Aggregate(FINAL)[regionkey] => [regionkey:bigint, count:bigint]
                count := "count"("count_8")
             - LocalExchange[HASH][$hashvalue] ("regionkey") => regionkey:bigint, count_8:bigint, $hashvalue:bigint
                 - RemoteExchange[REPARTITION][$hashvalue_9] => regionkey:bigint, count_8:bigint, $hashvalue_9:bigint
                     - Project[] => [regionkey:bigint, count_8:bigint, $hashvalue_10:bigint]
                             $hashvalue_10 := "combine_hash"(BIGINT '0', COALESCE("$operator$hash_code"("regionkey"), 0))
                         - Aggregate(PARTIAL)[regionkey] => [regionkey:bigint, count_8:bigint]
                                 count_8 := "count"(*)
                             - TableScan[tpch:tpch:nation:sf0.1, originalConstraint = true] => [regionkey:bigint]
                                     regionkey := tpch:regionkey
```

Distributed plan:

``` sql
lk:tiny> EXPLAIN (TYPE DISTRIBUTED) SELECT regionkey, count(*) FROM nation GROUP BY 1;
                                          Query Plan
----------------------------------------------------------------------------------------------
 Fragment 0 [SINGLE]
     Output layout: [regionkey, count]
     Output partitioning: SINGLE []
     - Output[regionkey, _col1] => [regionkey:bigint, count:bigint]
             _col1 := count
         - RemoteSource[1] => [regionkey:bigint, count:bigint]

 Fragment 1 [HASH]
     Output layout: [regionkey, count]
     Output partitioning: SINGLE []
     - Aggregate(FINAL)[regionkey] => [regionkey:bigint, count:bigint]
             count := "count"("count_8")
         - LocalExchange[HASH][$hashvalue] ("regionkey") => regionkey:bigint, count_8:bigint, $hashvalue:bigint
             - RemoteSource[2] => [regionkey:bigint, count_8:bigint, $hashvalue_9:bigint]

 Fragment 2 [SOURCE]
     Output layout: [regionkey, count_8, $hashvalue_10]
     Output partitioning: HASH [regionkey][$hashvalue_10]
     - Project[] => [regionkey:bigint, count_8:bigint, $hashvalue_10:bigint]
             $hashvalue_10 := "combine_hash"(BIGINT '0', COALESCE("$operator$hash_code"("regionkey"), 0))
         - Aggregate(PARTIAL)[regionkey] => [regionkey:bigint, count_8:bigint]
                 count_8 := "count"(*)
             - TableScan[tpch:tpch:nation:sf0.1, originalConstraint = true] => [regionkey:bigint]
                     regionkey := tpch:regionkey
```

Validate:

``` sql
lk:tiny> EXPLAIN (TYPE VALIDATE) SELECT regionkey, count(*) FROM nation GROUP BY 1;
 Valid
-------
 true
```

IO:

``` sql
lk:hive> EXPLAIN (TYPE IO, FORMAT JSON) INSERT INTO test_nation SELECT * FROM nation WHERE regionkey = 2;
            Query Plan
-----------------------------------
 {
   "inputTableColumnInfos" : [ {
     "table" : {
       "catalog" : "hive",
       "schemaTable" : {
         "schema" : "tpch",
         "table" : "nation"
       }
     },
     "columns" : [ {
       "columnName" : "regionkey",
       "type" : "bigint",
       "domain" : {
         "nullsAllowed" : false,
         "ranges" : [ {
           "low" : {
             "value" : "2",
             "bound" : "EXACTLY"
           },
           "high" : {
             "value" : "2",
             "bound" : "EXACTLY"
           }
         } ]
       }
     } ]
   } ],
   "outputTable" : {
     "catalog" : "hive",
     "schemaTable" : {
       "schema" : "tpch",
       "table" : "test_nation"
     }
   }
 }
```

See Also
--------

[EXPLAIN ANALYZE](./explain-analyze.md)
