EXPLAIN

Function

EXPLAIN shows the execution plan of an SQL statement.

The execution plan shows how the tables referenced by the SQL statement will be scanned, for example, by plain sequential scan or index scan. If multiple tables are referenced, the execution plan also shows what join algorithms will be used to bring together the required rows from each input table.

The most critical part of the display is the estimated statement execution cost, which is the planner's guess at how long it will take to run the statement.

The ANALYZE option causes the statement to be executed, not only planned. Then actual runtime statistics are added to the display, including the total elapsed time expended within each plan node (in milliseconds) and the total number of rows it actually returned. This is useful to check whether the planner's estimates are close to reality.

Precautions

The statement is executed when the ANALYZE option is used. To use EXPLAIN ANALYZE on an INSERT, UPDATE, DELETE, CREATE TABLE AS, or EXECUTE statement without letting the command affect your data, use this approach:

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START TRANSACTION;
EXPLAIN ANALYZE ...;
ROLLBACK;

Syntax

Parameter Description

Examples

Create the tpcds.customer_address_p1 table.

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CREATE TABLE tpcds.customer_address_p1 AS TABLE tpcds.customer_address;

Change the value of explain_perf_mode to normal.

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SET explain_perf_mode=normal;

Display an execution plan for simple queries in the table.

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EXPLAIN SELECT * FROM tpcds.customer_address_p1;
			       QUERY PLAN
----------------------------------------------------------------------------
 Data Node Scan on "__REMOTE_FQS_QUERY__"  (cost=0.00..0.00 rows=0 width=0)
   Node/s: All datanodes
(2 rows)

Generate an execution plan in JSON format (assume explain_perf_mode is set to normal).

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EXPLAIN(FORMAT JSON) SELECT * FROM tpcds.customer_address_p1;
                    QUERY PLAN
---------------------------------------------------
 [                                                +
   {                                              +
     "Plan": {                                    +
       "Node Type": "Data Node Scan",             +
       "RemoteQuery name": "__REMOTE_FQS_QUERY__",+
       "Alias": "__REMOTE_FQS_QUERY__",           +
       "Startup Cost": 0.00,                      +
       "Total Cost": 0.00,                        +
       "Plan Rows": 0,                            +
       "Plan Width": 0,                           +
       "Nodes": "All datanodes"                   +
     }                                            +
   }                                              +
 ]
(1 row)

If there is an index and we use a query with an indexable WHERE condition, EXPLAIN might show a different plan.

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EXPLAIN SELECT * FROM tpcds.customer_address_p1 WHERE ca_address_sk=10000;
                                  QUERY PLAN
------------------------------------------------------------------------------
 Data Node Scan on "__REMOTE_LIGHT_QUERY__"  (cost=0.00..0.00 rows=0 width=0)
   Node/s: datanode2
(2 rows)

Generate an execution plan in YAML format (assume explain_perf_mode is set to normal).

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EXPLAIN(FORMAT YAML) SELECT * FROM tpcds.customer_address_p1 WHERE ca_address_sk=10000;
                   QUERY PLAN
------------------------------------------------
 - Plan:                                       +
     Node Type: "Data Node Scan"               +
     RemoteQuery name: "__REMOTE_LIGHT_QUERY__"+
     Alias: "__REMOTE_LIGHT_QUERY__"           +
     Startup Cost: 0.00                        +
     Total Cost: 0.00                          +
     Plan Rows: 0                              +
     Plan Width: 0                             +
     Nodes: "datanode2"
(1 row)

Here is an example of an execution plan with cost estimates suppressed.

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EXPLAIN(COSTS FALSE)SELECT * FROM tpcds.customer_address_p1 WHERE ca_address_sk=10000;
                 QUERY PLAN
--------------------------------------------
 Data Node Scan on "__REMOTE_LIGHT_QUERY__"
   Node/s: datanode2
(2 rows)

Here is an example of an execution plan for a query that uses an aggregate function.

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EXPLAIN SELECT SUM(ca_address_sk) FROM tpcds.customer_address_p1 WHERE ca_address_sk<10000;
                                      QUERY PLAN                                       
---------------------------------------------------------------------------------------
 Aggregate  (cost=18.19..14.32 rows=1 width=4)
   ->  Streaming (type: GATHER)  (cost=18.19..14.32 rows=3 width=4)
         Node/s: All datanodes
         ->  Aggregate  (cost=14.19..14.20 rows=3 width=4)
               ->  Seq Scan on customer_address_p1  (cost=0.00..14.18 rows=10 width=4)
                     Filter: (ca_address_sk < 10000)
(6 rows)

Delete the tpcds.customer_address_p1 table.

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DROP TABLE tpcds.customer_address_p1;

Run the EXPLAIN PERFORMANCE command for the ANALYZE statement.

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EXPLAIN PERFORMANCE ANALYZE t2_dist_row;
                            QUERY EXEC INFO                            
-----------------------------------------------------------------------
 lock FirstCN: 
        coordinator1: actual time=0.240 loops=1
 estimate rows: actual time=[datanode3 0.000, datanode1 0.001]
        coordinator1: actual time=0.000 loops=1
        datanode1: actual time=0.001 loops=1
        datanode2: actual time=0.000 loops=1
        datanode3: actual time=0.000 loops=1
 sample rows: actual time=[datanode1 5.109, coordinator1 119.838]
        coordinator1: actual time=119.838 loops=1
        datanode1: actual time=5.109 loops=1
        datanode2: actual time=5.621 loops=1
        datanode3: actual time=5.342 loops=1
 fetch global stats: 
        coordinator1: actual time=8.501 loops=1
 calc stats: actual time=[datanode3 80.794, datanode2 109.155]
        coordinator1: actual time=97.452 loops=1
        datanode1: actual time=94.375 loops=1
        datanode2: actual time=109.155 loops=1
        datanode3: actual time=80.794 loops=1
 calc column stats: actual time=[datanode2 0.938, datanode3 9.811]
        coordinator1: actual time=5.162 loops=2
        datanode1: actual time=1.453 loops=2
        datanode2: actual time=0.938 loops=2
        datanode3: actual time=9.811 loops=2
 calc index stats: actual time=[datanode3 12.392, coordinator1 36.113]
        coordinator1: actual time=36.113 loops=1
        datanode1: actual time=15.933 loops=1
        datanode2: actual time=13.419 loops=1
        datanode3: actual time=12.392 loops=1
 calc expr stats: actual time=[datanode3 41.665, datanode2 78.442]
        coordinator1: actual time=55.608 loops=1
        datanode1: actual time=63.179 loops=1
        datanode2: actual time=78.442 loops=1
        datanode3: actual time=41.665 loops=1
 sync stats: 
        coordinator1: actual time=7.906 loops=1

 General Tracks
 CN build CN connection: 
        coordinator1: actual time=0.002 loops=1
 CN build DN connection: 
        coordinator1: actual time=0.070 loops=1
 -> execute ddl on other CN: 
        coordinator1: actual time=0.001 loops=1
 -> execute ddl on other DN: 
        coordinator1: actual time=0.000 loops=1
 Query Id: 72902018968225366
 Total runtime: 242.211 ms
(48 rows)

Show the blockname of a plan.

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EXPLAIN (BLOCKNAME ON) SELECT SUM(ca_address_sk) FROM tpcds.customer_address_p1 WHERE ca_address_sk<10000;
                                         QUERY PLAN
---------------------------------------------------------------------------------------------
  id |                  operation                   | E-rows | E-memory | E-width | E-costs
 ----+----------------------------------------------+--------+----------+---------+---------
   1 | ->  Aggregate                                |      1 |          |      12 | 16.14
   2 |    ->  Streaming (type: GATHER)              |      2 |          |      12 | 16.14
   3 |       ->  Aggregate                          |      2 | 1MB      |      12 | 10.14
   4 |          ->  Seq Scan on customer_address_p1 |      7 | 1MB      |       4 | 10.12

 Predicate Information (identified by plan id)
 ---------------------------------------------
   4 --Seq Scan on customer_address_p1
         Filter: (ca_address_sk < 10000)

 Query Block Name / Object Alias (identified by plan id)
 -------------------------------------------------------
   1 - sel$1
   4 - sel$1 / customer_address_p1@"sel$1"

   ====== Query Summary =====
 -------------------------------
 System available mem: 4710400KB
 Query Max mem: 4710400KB
 Query estimated mem: 2048KB
(22 rows)

Show the outline of a plan.

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EXPLAIN (OUTLINE ON) SELECT SUM(ca_address_sk) FROM tpcds.customer_address_p1 WHERE ca_address_sk<10000;
                                         QUERY PLAN
---------------------------------------------------------------------------------------------
  id |                  operation                   | E-rows | E-memory | E-width | E-costs
 ----+----------------------------------------------+--------+----------+---------+---------
   1 | ->  Aggregate                                |      1 |          |      12 | 16.14
   2 |    ->  Streaming (type: GATHER)              |      2 |          |      12 | 16.14
   3 |       ->  Aggregate                          |      2 | 1MB      |      12 | 10.14
   4 |          ->  Seq Scan on customer_address_p1 |      7 | 1MB      |       4 | 10.12

 Predicate Information (identified by plan id)
 ---------------------------------------------
   4 --Seq Scan on customer_address_p1
         Filter: (ca_address_sk < 10000)

                         Outline Data
 ------------------------------------------------------------
   /*+
       begin_outline_data
        TableScan(@"sel$1" tpcds.customer_address_p1@"sel$1")
       end_outline_data
   */

   ====== Query Summary =====
 -------------------------------
 System available mem: 4710400KB
 Query Max mem: 4710400KB
 Query estimated mem: 2048KB
(25 rows)

Helpful Links

ANALYZE | ANALYSE

Parent topic: DML Syntax