Purpose
You can use this statement to obtain the execution plan of an SQL statement, which can be a SELECT, DELETE, INSERT, REPLACE or UPDATE statement.
EXPLAIN, DESCRIBE, and DESC are synonyms.
Syntax
/*Obtain table or column information.*/
{EXPLAIN | DESCRIBE | DESC} table_name [column_name];
/*Obtain the SQL plan.*/
{EXPLAIN [INTO table_name ] [SET statement_id = string]
| DESCRIBE
| DESC} [explain_type] [PRETTY | PRETTY_COLOR] dml_statement;
explain_type:
BASIC
| OUTLINE
| EXTENDED
| EXTENDED_NOADDR
| PARTITIONS
| FORMAT = {TRADITIONAL| JSON}
dml_statement:
SELECT statement
| DELETE statement
| INSERT statement
| UPDATE statement
| MERGE statement
Parameters
Parameter |
Description |
|---|---|
| table_name | The table name. |
| column_name | The column name of the table. |
| INTO table_name | The table where the execution plan obtained by EXPLAIN is to be stored. If you do not specify INTO table_name, the execution plan is stored in the PLAN_TABLE table by default. |
| SET statement_id | The statement ID of the explained SQL statement, which can be used to query the execution plan of the statement later. If you do not specify SET statement_id, an empty string is used as the statement ID by default. |
| PRETTY | PRETTY_COLOR | Specifies to connect the parent and child nodes in the plan tree with tree lines or colored tree lines to make the execution plan easier to read. |
| BASIC | The basic information about the output plan, such as the operator ID, operator name, and name of the referenced table. |
| explain_type | The explanation type. |
| BASIC | Outputs basic information about the execution plan, such as the operator ID, operator name, and name of the referenced table. |
| OUTLINE | Outputs execution plan information, with the outline included. |
| EXTENDED | The extra information generated by EXPLAIN, including the input and output columns of each operator, partition information about the access table, and the active filter. If the current operator uses an index, the used index columns and extracted query range are displayed. If the current operator references an index, the referenced index and the extracted query range are displayed in the information. |
| EXTENDED_NOADDR | Specifies to display the brief extra information. |
| PARTITIONS | Specifies to display the partition-related information. |
| TRADITIONAL | JSON | The output format of EXPLAIN. Valid values:
|
| dml_statement | The DML statement. |
Examples
Explain an SQL query and store the obtained execution plan in the
testtable./* Create a table named test. */ obclient> CREATE TABLE test (statement_id VARCHAR(30), plan_id INT, gmt_create TIMESTAMP, remarks VARCHAR(4000), operator VARCHAR(255), options VARCHAR(255), object_node VARCHAR(40), object_owner VARCHAR(128), object_name VARCHAR(128), object_alias VARCHAR(261), object_instance INT, object_type VARCHAR(30),optimizer VARCHAR(4000), search_columns INT, id INT, parent_id INT, depth INT, position INT,is_last_child INT, cost INT, cardinality INT, bytes INT, rowset INT, other_tag VARCHAR(4000), partition_start VARCHAR(4000), partition_stop VARCHAR(4000), partition_id INT, other VARCHAR(4000), distribution VARCHAR(64), cpu_cost INT, io_cost INT, temp_space INT, access_predicates VARCHAR(4000), filter_predicates VARCHAR(4000),startup_predicates VARCHAR(4000), projection VARCHAR(4000), special_predicates VARCHAR(4000), time INT, qblock_name VARCHAR(128), other_xml VARCHAR(4000)); Query OK, 0 rows affected obclient> CREATE TABLE t1(c1 INT); Query OK, 0 rows affected /* If the time format is incorrect, run the following command to reset the TIMESTAMP format. */ obclient> ALTER SESSION SET NLS_TIMESTAMP_FORMAT = 'YYYY-MM-DD HH24:MI:SS.FF'; Query OK, 0 rows affected /* Explain an SQL query and store the obtained execution plan in the test table. */ obclient> EXPLAIN INTO test SELECT COUNT(*) FROM t1; +--------------------------------------------------------------------------+ | Query Plan | +--------------------------------------------------------------------------+ | ================================================= | | |ID|OPERATOR |NAME|EST.ROWS|EST.TIME(us)| | | ------------------------------------------------- | | |0 |SCALAR GROUP BY | |1 |2 | | | |1 |└─TABLE FULL SCAN|T1 |1 |2 | | | ================================================= | | Outputs & filters: | | ------------------------------------- | | 0 - output([T_FUN_COUNT_SUM(T_FUN_COUNT(*))]), filter(nil), rowset=256 | | group(nil), agg_func([T_FUN_COUNT_SUM(T_FUN_COUNT(*))]) | | 1 - output([T_FUN_COUNT(*)]), filter(nil), rowset=256 | | access(nil), partitions(p0) | | is_index_back=false, is_global_index=false, | | range_key([T1.__pk_increment]), range(MIN ; MAX)always true | +--------------------------------------------------------------------------+ 14 rows in set obclient> SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY(table_name=>'test')); +--------------------------------------------------------------------------+ | COLUMN_VALUE | +--------------------------------------------------------------------------+ | ================================================= | | |ID|OPERATOR |NAME|EST.ROWS|EST.TIME(us)| | | ------------------------------------------------- | | |0 |SCALAR GROUP BY | |1 |2 | | | |1 |└─TABLE FULL SCAN|T1 |1 |2 | | | ================================================= | | Outputs & filters: | | ------------------------------------- | | 0 - output([T_FUN_COUNT_SUM(T_FUN_COUNT(*))]), filter(nil), rowset=256 | | group(nil), agg_func([T_FUN_COUNT_SUM(T_FUN_COUNT(*))]) | | 1 - output([T_FUN_COUNT(*)]), filter(nil), rowset=256 | | access(nil), partitions(p0) | | is_index_back=false, is_global_index=false, | | range_key([T1.__pk_increment]), range(MIN ; MAX)always true | +--------------------------------------------------------------------------+ 14 rows in setExplain an SQL query and set an ID for the query.
obclient> EXPLAIN INTO test SET statement_id='test2' SELECT COUNT(*) FROM t1; +--------------------------------------------------------------------------+ | Query Plan | +--------------------------------------------------------------------------+ | ================================================= | | |ID|OPERATOR |NAME|EST.ROWS|EST.TIME(us)| | | ------------------------------------------------- | | |0 |SCALAR GROUP BY | |1 |2 | | | |1 |└─TABLE FULL SCAN|T1 |1 |2 | | | ================================================= | | Outputs & filters: | | ------------------------------------- | | 0 - output([T_FUN_COUNT_SUM(T_FUN_COUNT(*))]), filter(nil), rowset=256 | | group(nil), agg_func([T_FUN_COUNT_SUM(T_FUN_COUNT(*))]) | | 1 - output([T_FUN_COUNT(*)]), filter(nil), rowset=256 | | access(nil), partitions(p0) | | is_index_back=false, is_global_index=false, | | range_key([T1.__pk_increment]), range(MIN ; MAX)always true | +--------------------------------------------------------------------------+ 14 rows in set obclient> SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY(table_name=>'test',statement_id=>'test2')); +--------------------------------------------------------------------------+ | COLUMN_VALUE | +--------------------------------------------------------------------------+ | ================================================= | | |ID|OPERATOR |NAME|EST.ROWS|EST.TIME(us)| | | ------------------------------------------------- | | |0 |SCALAR GROUP BY | |1 |2 | | | |1 |└─TABLE FULL SCAN|T1 |1 |2 | | | ================================================= | | Outputs & filters: | | ------------------------------------- | | 0 - output([T_FUN_COUNT_SUM(T_FUN_COUNT(*))]), filter(nil), rowset=256 | | group(nil), agg_func([T_FUN_COUNT_SUM(T_FUN_COUNT(*))]) | | 1 - output([T_FUN_COUNT(*)]), filter(nil), rowset=256 | | access(nil), partitions(p0) | | is_index_back=false, is_global_index=false, | | range_key([T1.__pk_increment]), range(MIN ; MAX)always true | +--------------------------------------------------------------------------+ 14 rows in setCreate tables
t101andt102.obclient> CREATE TABLE t101(c1 INT,c2 INT); Query OK, 0 rows affected obclient> CREATE TABLE t102(c1 INT,c2 INT); Query OK, 0 rows affectedObtain the information about the
tbl1table.obclient> EXPLAIN t101; +-------+------------+------+-----+---------+-------+ | FIELD | TYPE | NULL | KEY | DEFAULT | EXTRA | +-------+------------+------+-----+---------+-------+ | C1 | NUMBER(38) | YES | NULL | NULL | NULL | | C2 | NUMBER(38) | YES | NULL | NULL | NULL | +-------+------------+------+-----+---------+-------+ 2 rows in setObtain the information about the
c2column of thet102table.obclient> EXPLAIN t102 c2; +-------+------------+------+-----+---------+-------+ | FIELD | TYPE | NULL | KEY | DEFAULT | EXTRA | +-------+------------+------+-----+---------+-------+ | C2 | NUMBER(38) | YES | NULL | NULL | NULL | +-------+------------+------+-----+---------+-------+ 1 row in setOmit
explain_typeand return the execution plan of theSELECTstatement.+------------------------------------------------------------------------------------+ | Query Plan | +------------------------------------------------------------------------------------+ | ================================================= | | |ID|OPERATOR |NAME|EST.ROWS|EST.TIME(us)| | | ------------------------------------------------- | | |0 |HASH JOIN | |1 |8 | | | |1 |├─TABLE FULL SCAN|T102|1 |4 | | | |2 |└─TABLE FULL SCAN|T101|1 |4 | | | ================================================= | | Outputs & filters: | | ------------------------------------- | | 0 - output([T101.C1], [T101.C2], [T102.C1], [T102.C2]), filter(nil), rowset=16 | | equal_conds([T101.C2 = T102.C2]), other_conds(nil) | | 1 - output([T102.C2], [T102.C1]), filter([T102.C1 > 4]), rowset=16 | | access([T102.C2], [T102.C1]), partitions(p0) | | is_index_back=false, is_global_index=false, filter_before_indexback[false], | | range_key([T102.__pk_increment]), range(MIN ; MAX)always true | | 2 - output([T101.C2], [T101.C1]), filter(nil), rowset=16 | | access([T101.C2], [T101.C1]), partitions(p0) | | is_index_back=false, is_global_index=false, | | range_key([T101.__pk_increment]), range(MIN ; MAX)always true | +------------------------------------------------------------------------------------+Use the
EXTENDED_NOADDRkeyword to output additional information in the execution plan.obclient> EXPLAIN EXTENDED_NOADDR SELECT * FROM t101,t102 WHERE t101.c2=t102.c2 AND t102.c1 > 4; +------------------------------------------------------------------------------------+ | Query Plan | +------------------------------------------------------------------------------------+ | ================================================= | | |ID|OPERATOR |NAME|EST.ROWS|EST.TIME(us)| | | ------------------------------------------------- | | |0 |HASH JOIN | |1 |8 | | | |1 |├─TABLE FULL SCAN|T102|1 |4 | | | |2 |└─TABLE FULL SCAN|T101|1 |4 | | | ================================================= | | Outputs & filters: | | ------------------------------------- | | 0 - output([T101.C1], [T101.C2], [T102.C1], [T102.C2]), filter(nil), rowset=16 | | equal_conds([T101.C2 = T102.C2]), other_conds(nil) | | 1 - output([T102.C2], [T102.C1]), filter([T102.C1 > 4]), rowset=16 | | access([T102.C2], [T102.C1]), partitions(p0) | | is_index_back=false, is_global_index=false, filter_before_indexback[false], | | range_key([T102.__pk_increment]), range(MIN ; MAX)always true | | 2 - output([T101.C2], [T101.C1]), filter(nil), rowset=16 | | access([T101.C2], [T101.C1]), partitions(p0) | | is_index_back=false, is_global_index=false, | | range_key([T101.__pk_increment]), range(MIN ; MAX)always true | | Used Hint: | | ------------------------------------- | | /*+ | | | | */ | | Qb name trace: | | ------------------------------------- | | stmt_id:0, stmt_type:T_EXPLAIN | | stmt_id:1, SEL$1 | | Outline Data: | | ------------------------------------- | | /*+ | | BEGIN_OUTLINE_DATA | | LEADING(@"SEL$1" ("SYS"."T102"@"SEL$1" "SYS"."T101"@"SEL$1")) | | USE_HASH(@"SEL$1" "SYS"."T101"@"SEL$1") | | FULL(@"SEL$1" "SYS"."T102"@"SEL$1") | | FULL(@"SEL$1" "SYS"."T101"@"SEL$1") | | OPTIMIZER_FEATURES_ENABLE('4.0.0.0') | | END_OUTLINE_DATA | | */ | | Optimization Info: | | ------------------------------------- | | T102: | | table_rows:1 | | physical_range_rows:1 | | logical_range_rows:1 | | index_back_rows:0 | | output_rows:0 | | table_dop:1 | | dop_method:Table DOP | | avaiable_index_name:[T102] | | stats version:0 | | dynamic sampling level:1 | | T101: | | table_rows:1 | | physical_range_rows:1 | | logical_range_rows:1 | | index_back_rows:0 | | output_rows:1 | | table_dop:1 | | dop_method:Table DOP | | avaiable_index_name:[T101] | | stats version:0 | | dynamic sampling level:1 | | Plan Type: | | LOCAL | | Note: | | Degree of Parallelisim is 1 because of table property | +------------------------------------------------------------------------------------+Show the execution plan of the
INSERTstatement inTRADITIONALformat.obclient> EXPLAIN FORMAT=TRADITIONAL INSERT INTO T101 VALUES(1,1); +----------------------------------------------------------------------------------------------------------------------------------------+ | Query Plan | +----------------------------------------------------------------------------------------------------------------------------------------+ | ================================================== | | |ID|OPERATOR |NAME|EST.ROWS|EST.TIME(us)| | | -------------------------------------------------- | | |0 |DISTRIBUTED INSERT| |1 |13 | | | |1 |└─EXPRESSION | |1 |1 | | | ================================================== | | Outputs & filters: | | ------------------------------------- | | 0 - output(nil), filter(nil) | | columns([{T101: ({T101: (T101.__pk_increment, T101.C1, T101.C2)})}]), | | column_values([T_HIDDEN_PK], [column_conv(NUMBER,PS:(-1,0),NULL,__values.C1)], [column_conv(NUMBER,PS:(-1,0),NULL,__values.C2)]) | | 1 - output([__values.C1], [__values.C2]), filter(nil) | | values({1, 1}) | +----------------------------------------------------------------------------------------------------------------------------------------+Show the execution plan of the
SELECTstatement inJSONformat.obclient> EXPLAIN FORMAT=JSON SELECT * FROM t101,t102 WHERE t101.c2=t102.c2 AND t102.c1 > 4; +------------------------------------------------------------------+ | Query Plan | +------------------------------------------------------------------+ | { | | "ID":0, | | "OPERATOR":"HASH JOIN ", | | "NAME":"", | | "EST.ROWS":1, | | "EST.TIME(us)":8, | | "output":"output([T101.C1], [T101.C2], [T102.C1], [T102.C2])", | | "CHILD_1": { | | "ID":1, | | "OPERATOR":"TABLE FULL SCAN", | | "NAME":"T102", | | "EST.ROWS":1, | | "EST.TIME(us)":4, | | "output":"output([T102.C2], [T102.C1])" | | }, | | "CHILD_2": { | | "ID":2, | | "OPERATOR":"TABLE FULL SCAN", | | "NAME":"T101", | | "EST.ROWS":1, | | "EST.TIME(us)":4, | | "output":"output([T101.C2], [T101.C1])" | | } | | } | +------------------------------------------------------------------+