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

• Display the execution plan of an SQL statement, which supports multiple options and has no requirements for the order of options.

  1	EXPLAIN [ ( option [, ...] ) ] statement;

  The syntax of the option clause is as follows:

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  ANALYZE [ boolean ] | ANALYSE [ boolean ] | VERBOSE [ boolean ] | COSTS [ boolean ] | CPU [ boolean ] | DETAIL [ boolean ] | NODES [ boolean ] | NUM_NODES [ boolean ] | BUFFERS [ boolean ] | TIMING [ boolean ] | PLAN [ boolean ] | FORMAT { TEXT | XML | JSON | YAML }

• Display the execution plan of an SQL statement, where options are in order.

  1	EXPLAIN { [ { ANALYZE | ANALYSE } ] [ VERBOSE ] | PERFORMANCE } statement;

• Display information required for reproducing the execution plan of an SQL statement. The information is usually used for fault locating. The STATS option must be used independently.

  1	EXPLAIN ( STATS [ boolean ] ) statement;

Parameter Description

• statement

  Specifies the SQL statement to explain.

• ANALYZE boolean | ANALYSE boolean

  Displays the actual run times and other statistics.

  Valid value:

  • TRUE (default value): Displays the actual run times and other statistics.
  • FALSE: No display.
• VERBOSE boolean

  Displays additional information regarding the plan.

  Valid value:

  • TRUE (default value): Displays additional information.
  • FALSE: No display.
• COSTS boolean

  Includes information on the estimated total cost of each plan node, as well as the estimated number of rows and the estimated width of each row.

  Valid value:

  • TRUE (default): Displays information on the estimated total cost of each plan node and the estimated width of each row.
  • FALSE: No display.
• CPU boolean

  Prints information on CPU usage.

  Valid value:

  • TRUE (default value): Displays CPU usage information.
  • FALSE: No display.
• DETAIL boolean

  Prints DN information.

  Valid value:

  • TRUE (default value): Prints DN information.
  • FALSE: No display.
  

  In 8.2.1 and later cluster versions, if Detail is enabled in EXPLAIN, the skew value comparison time is displayed in the execution plan.

• NODES boolean

  Prints information about the nodes executed by query.

  Valid value:

  • TRUE (default): Prints information about executed nodes.
  • FALSE: No display.
• NUM_NODES boolean

  Prints the quantity of executing nodes.

  Valid value:

  • TRUE (default value): Prints the number of DNs.
  • FALSE: No display.
• BUFFERS boolean

  Includes information on buffer usage.

  Valid value:

  • TRUE: Displays information on buffer usage.
  • FALSE (default): No display.
• TIMING boolean

  Includes the startup time and the time spent on the output node.

  Valid value:

  • TRUE (Default): Displays the startup time and the time spent on the output node.
  • FALSE: No display.
• PLAN

  Specifies whether to store the execution plan in PLAN_TABLE. If this parameter is set to on, the execution plan is stored in PLAN_TABLE and is not displayed on the screen. Therefore, this parameter cannot be used together with other parameters when it is set to on.

  Valid value:

  • on: The execution plan is stored in PLAN_TABLE and is not printed on the screen. It is the default value. If the plan is stored successfully, EXPLAIN SUCCESS is returned.
  • off: The execution plan is not stored in PLAN_TABLE and is printed on the screen.
• FORMAT

  Specifies the output format.

  Value range: TEXT, XML, JSON, and YAML.

  Default value: TEXT

• PERFORMANCE

  This option prints all relevant information in execution.

• STATS boolean

  Specifies whether to display information required for reproducing the execution plan of an SQL statement, including the object definition, statistics, and configuration parameters. The information is usually used for fault locating.

  Valid value:

  • TRUE (default value): Display information required for reproducing the execution plan of an SQL statement.
  • FALSE: No display.

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;

Helpful Links

ANALYZE | ANALYSE