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EXPLAIN

Function

EXPLAIN shows the execution plan of an SQL statement.

The execution plan shows how the tables referenced by the statement will be scanned - by plain sequential scan, index scan, etc. - and if multiple tables are referenced, 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 actually executed, not only planned. The total elapsed time expended within each plan node (in milliseconds) and total number of rows it actually returned are added to the display. This is useful for seeing whether the planner's estimates are close to reality.

Precautions

  • The statement is actually executed when the ANALYZE option is used. If you want to use EXPLAIN to analyze INSERT, UPDATE, DELETE, CREATE TABLE AS, or EXECUTE statement without letting the statement affect your data, use this approach:

    START TRANSACTION;
    EXPLAIN ANALYZE ...;
    ROLLBACK;
  • The NODES and NUM_NODES parameters are disabled in standalone mode because they are available only in distributed mode. If parameters are used, the following error is reported:

    mogdb=# create table student(id int, name char(20));
    CREATE TABLE
    mogdb=# explain (nodes true) insert into student values(5,'a'),(6,'b');
    ERROR:  unrecognized EXPLAIN option "nodes"
    mogdb=# explain (num_nodes true) insert into student values(5,'a'),(6,'b');
    ERROR:  unrecognized EXPLAIN option "num_nodes"

Syntax

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

    Explain ::= EXPLAIN [ (  option  [, ...] )  ] statement;

    The syntax of the option clause is as follows:

    option ::= 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.

    Explain ::= EXPLAIN  { [  { ANALYZE  | ANALYSE  }  ] [ VERBOSE  ]  | PERFORMANCE  } statement;

Parameter Description

  • statement

    Specifies the SQL statement to explain.

  • ANALYZE boolean | ANALYSE boolean

    Specifies whether to display actual run times and other statistics.

    Value range:

    • TRUE (default): displays them.
    • FALSE: does not display them.
  • VERBOSE boolean

    Specifies whether to display additional information regarding the plan.

    Value range:

    • TRUE (default): displays them.
    • FALSE: does not display them.
  • COSTS boolean

    Specifies whether to display the estimated total cost of each plan node, estimated number of rows, estimated width of each row.

    Value range:

    • TRUE (default): displays them.
    • FALSE: does not display them.
  • CPU boolean

    Specifies whether to display CPU usage.

    Value range:

    • TRUE (default): displays it.
    • FALSE: does not display it.
  • DETAIL boolean

    Displays information about database nodes.

    Value range:

    • TRUE (default value): displays it.
    • FALSE: does not display it.
  • NODES boolean

    Specifies whether to display information about the nodes executed by query.

    Value range:

    • TRUE (default): displays it.
    • FALSE: does not display it.
  • NUM_NODES boolean

    Specifies whether to display the number of executing nodes.

    Value range:

    • TRUE (default value): displays it.
    • FALSE: does not display it.
  • BUFFERS boolean

    Specifies whether to display buffer usage.

    Value range:

    • TRUE (default): displays it.
    • FALSE: does not display it.
  • TIMING boolean

    Specifies whether to display the actual startup time and time spent on the output node.

    Value range:

    • TRUE (default): displays them.
    • FALSE: does not display them.
  • 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 not displayed on the screen. Therefore, this parameter cannot be used together with other parameters when it is set to on.

    Value range:

    • ON (default): The execution plan is stored in PLAN_TABLE and not printed on the screen. If the plan is stored successfully, “EXPLAIN SUCCESS” is returned.
    • OFF: The execution plan is not stored in PLAN_TABLE but is printed on the screen.
  • FORMAT

    Specifies the output format.

    Value range: TEXT, XML, JSON, and YAML

    Default value: TEXT

  • PERFORMANCE

    Prints all relevant information in execution.

Examples

-- Create the tpcds.customer_address_p1 table.
mogdb=# CREATE TABLE tpcds.customer_address_p1 AS TABLE tpcds.customer_address;

-- Change the value of explain_perf_mode to normal.
mogdb=# SET explain_perf_mode=normal;

-- Display an execution plan for simple queries in the table.
mogdb=# EXPLAIN SELECT * FROM tpcds.customer_address_p1;
QUERY PLAN
--------------------------------------------------
Data Node Scan  (cost=0.00..0.00 rows=0 width=0)
Node/s: All dbnodes
(2 rows)

-- Generate an execution plan in JSON format (with explain_perf_mode being normal).
mogdb=# EXPLAIN(FORMAT JSON) SELECT * FROM tpcds.customer_address_p1;
              QUERY PLAN              
--------------------------------------
 [                                   +
   {                                 +
     "Plan": {                       +
       "Node Type": "Data Node Scan",+
       "Startup Cost": 0.00,         +
       "Total Cost": 0.00,           +
       "Plan Rows": 0,               +
       "Plan Width": 0,              +
       "Node/s": "All dbnodes"     +
     }                               +
   }                                 +
 ]
(1 row)

-- If there is an index and we use a query with an indexable WHERE condition, EXPLAIN might show a different plan.
mogdb=# EXPLAIN SELECT * FROM tpcds.customer_address_p1 WHERE ca_address_sk=10000;
QUERY PLAN
--------------------------------------------------
Data Node Scan  (cost=0.00..0.00 rows=0 width=0)
Node/s: dn_6005_6006
(2 rows)

-- Generate an execution plan in YAML format (with explain_perf_mode being normal).
mogdb=# EXPLAIN(FORMAT YAML) SELECT * FROM tpcds.customer_address_p1 WHERE ca_address_sk=10000;
           QUERY PLAN            
---------------------------------
 - Plan:                        +
     Node Type: "Data Node Scan"+
     Startup Cost: 0.00         +
     Total Cost: 0.00           +
     Plan Rows: 0               +
     Plan Width: 0              +
     Node/s: "dn_6005_6006"
(1 row)

-- Here is an example of a query plan with cost estimates suppressed:
mogdb=# EXPLAIN(COSTS FALSE)SELECT * FROM tpcds.customer_address_p1 WHERE ca_address_sk=10000;
       QUERY PLAN       
------------------------
 Data Node Scan
   Node/s: dn_6005_6006
(2 rows)

-- Here is an example of a query plan for a query using an aggregate function:
mogdb=# 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 dbnodes
         ->  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.
mogdb=# DROP TABLE tpcds.customer_address_p1;