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 DETAIL, NODES, and NUM_NODES parameters are disabled in standalone mode. They are available only in distributed mode. If the 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 it.
- FALSE: does not display it.
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): 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): displays it.
- FALSE: does not display it.
BUFFERS boolean

Specifies whether to display buffer usage.

Value range:
- TRUE: displays it.
- FALSE(default): 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)

-- Create a level-2 partitioned table.
MogDB=# CREATE TABLE range_list
MogDB-# (
MogDB(#     month_code VARCHAR2 ( 30 ) NOT NULL ,
MogDB(#     dept_code  VARCHAR2 ( 30 ) NOT NULL ,
MogDB(#     user_no    VARCHAR2 ( 30 ) NOT NULL ,
MogDB(#     sales_amt  int
MogDB(# )
MogDB-# PARTITION BY RANGE (month_code) SUBPARTITION BY LIST (dept_code)
MogDB-# (
MogDB(#   PARTITION p_201901 VALUES LESS THAN( '201903' )
MogDB(#   (
MogDB(#     SUBPARTITION p_201901_a values ('1'),
MogDB(#     SUBPARTITION p_201901_b values ('2')
MogDB(#   ),
MogDB(#   PARTITION p_201902 VALUES LESS THAN( '201910' )
MogDB(#   (
MogDB(#     SUBPARTITION p_201902_a values ('1'),
MogDB(#     SUBPARTITION p_201902_b values ('2')
MogDB(#   )
MogDB(# );
CREATE TABLE

-- Run a query statement containing a level-2 partitioned table.
-- Iterations and Sub Iterations specifies the numbers of level-1 and level-2 partitions that are traversed, respectively.
-- Selected Partitions specifies which level-1 partitions are actually scanned. Selected Subpartitions (p:s) indicates that s level-2 partitions under the pth level-1 partition are actually scanned. If all level-2 partitions under the level-1 partition are scanned, the value of s is ALL.
MogDB=# EXPLAIN SELECT * FROM range_list WHERE dept_code = '1';
                                  QUERY PLAN
-------------------------------------------------------------------------------
 Partition Iterator  (cost=0.00..13.81 rows=2 width=238)
   Iterations: 2, Sub Iterations: 2
   ->  Partitioned Seq Scan on range_list  (cost=0.00..13.81 rows=2 width=238)
         Filter: ((dept_code)::text = '1'::text)
         Selected Partitions:  1..2
         Selected Subpartitions:  1:1, 2:1
(6 rows)

-- Delete the tpcds.customer_address_p1 table.
MogDB=# DROP TABLE tpcds.customer_address_p1;

Helpful Links

ANALYZE | ANALYSE

Issue