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EXPLAIN
Returns detailed information on how the query is executed.
EXPLAIN displays query execution operations as a tree, where the operator at the top is the last executed before returning the result to the client, and operators below are executed before their parents. EXPLAIN plans can also be visualized via a graphical interface using SingleStore DB Studio Visual Explain.
See the examples section below for descriptions of the most common types of information in EXPLAIN output.
Syntax
EXPLAIN [EXTENDED | JSON] [INTO OUTFILE file_name]
{ select_statement
| insert_select_statement
| update_statement
| delete_statement }
Remarks
-
EXPLAIN can be run with an arbitrary SELECT, INSERT…SELECT, UPDATE, or DELETE statement.
-
EXPLAIN displays the plan MemSQL would generate if the query were reoptimized and recompiled now. If the query has already been run and a plan has already been saved in the plancache, that plan will be reused, even though it may differ from the plan displayed by EXPLAIN. You may run DROP … FROM PLANCACHE to force the query to be reoptimized and recompiled. Note that running ANALYZE will automatically drop and recompile query plans if data statistics have changed significantly.
-
In most cases when the optimizer makes a cost-based decision,
EXPLAINoutput displays estimated statistics and costs. -
If the
enable_disk_plan_explainglobal variable is enabled, the explain tree information is also stored in the persisted plancache, in human readable format. Whenenable_disk_plan_explainis enabled, theSHOW PLAN [JSON] plan_idcommand can also be used to display the explain information. Refer to the SHOW PLAN topic for details. -
The
INTO OUTFILE file_nameclause writes the explain information in JSON or text format to a file. It creates a new file at the specified location and writes the query profile details to the file. Enter the absolute path to the file as thefile_nameparameter. The output file can be a JSON or a text file.---To display the explain information in text format memsql> SHOW PLAN 3; ---To display the explain information in JSON format memsql> SHOW PLAN JSON 3;
Explain Modes
EXPLAINoutputs the explain information in a human readable formatEXPLAIN EXTENDEDoutputs more detailed information for the query plan in addition to the information displayed byEXPLAINEXPLAIN JSONoutputs the explain information in JSON format, which may be more machine readable
Additionally, the following EXPLAIN modes are provided for informational and debugging purposes only and may be changed at any time:
EXPLAIN MBCoutputs MemSQL Byte Code generated for the queryEXPLAIN MPLoutputs MemSQL Programming Language code generated for the query
EXPLAIN
EXPLAIN SELECT * FROM t;
****
+-----------------------------------+
| EXPLAIN |
+-----------------------------------+
| Project [t.id, t.a, t.b] |
| Gather partitions:all |
| Project [t.id, t.a, t.b] |
| TableScan db1.t, PRIMARY KEY (id) |
+-----------------------------------+
EXPLAIN EXTENDED
EXPLAIN EXTENDED SELECT * FROM t;
****
+----------------------------------------------------------------------------------------------------------------+
| EXPLAIN |
+----------------------------------------------------------------------------------------------------------------+
| Project [t.id, t.a, t.b] |
| Gather partitions:all query:[SELECT `t`.`id` AS `id`, `t`.`a` AS `a`, `t`.`b` AS `b` FROM `db1_0`.`t` as `t` ] |
| Project [t.id, t.a, t.b] |
| TableScan db1.t, PRIMARY KEY (id) |
+----------------------------------------------------------------------------------------------------------------+
EXPLAIN JSON
EXPLAIN JSON SELECT * FROM t;
****
----------------------------------------------
{
"executor":"Project",
"out":{
"":"t.id",
"":"t.a",
"":"t.b"
},
"inputs":[
{
"executor":"Gather",
"partitions":"all",
"query":"SELECT `t`.`id` AS `id`, `t`.`a` AS `a`, `t`.`b` AS `b` FROM `db1_0`.`t` as `t` ",
"inputs":[
{
"executor":"Project",
"out":{
"":"t.id",
"":"t.a",
"":"t.b"
},
"subselects":[],
"inputs":[
{
"executor":"TableScan",
"db":"db1",
"table":"t",
"alias":"t",
"index":"PRIMARY KEY (id)",
"storage":"lf_skiplist",
"dir":"forward",
"inputs":[]
}
]
}
]
}
]
}
EXPLAIN MBC
EXPLAIN MBC SELECT * FROM t;
****
----------------------------------------------
EXPLAIN
# explain mbc select * from t
Data:
static: offset=0 size=125 align=1 ata="USING `\?`::`\?` AGGREGATOR_ID = \?, CONNECTION_ID = \? SELECT `t`.`id` AS `id`, `t`.`a` AS `a`, `t`.`b` AS `b` FROM `t` as `t` \x00"""
Function 0 <100000:CreateProjectState>:
Locals:
local rvToReturn: offset=0 size=8 align=8 type=int64_t
local rvToReceive: offset=16 size=16 align=16 type=uint128_t
param hiddenRvParam: offset=32 size=8 align=8 type={ state1: { RemoteElement: RemoteElement, a: { value: int32_t, opt: Opt, pack(4) }, b: { value: int32_t, opt: Opt, pack(4) }, : int64_t, id: int32_t } }*
| local state1Addr_1: offset=40 size=8 align=8 type={ RemoteElement: RemoteElement, a: { value: int32_t, opt: Opt, pack(4) }, b: { value: int32_t, opt: Opt, pack(4) }, : int64_t, id: int32_t }*
0x0000 Junk local=hiddenRvParam i32=32
0x000c Lea local=&state1Addr_1 local=hiddenRvParam i32=0
0x001c Call func=10 <row_0tmp1_internal_ctor> local=state1Addr_1
0x002c Junk local=&state1Addr_1 i32=8
0x0038 Literal8 local=&rvToReturn i64=1
0x0048 Return
Function 1 <100000:CreateMatchedRowContext>:
........
Example - EXPLAIN JSON INTO OUTFILE
EXPLAIN JSON INTO OUTFILE '/tmp/t1.json' SELECT * FROM t1;
To view the contents of the t1.json file, run the following command at the root prompt:
cat '/tmp/t1.json';
Output file:
{
"plan_warnings": {
},
"explain":[
{
"executor":"Gather",
"partitions":"all",
"query":"SELECT `t1`.`a` AS `a`, `t1`.`b` AS `b` FROM `db_0`.`t1` as `t1` \/*!90623 OPTION(NO_QUERY_REWRITE=1, INTERPRETER_MODE=INTERPRET_FIRST)*\/",
"alias":"remote_0",
"inputs":[
{
"executor":"Project",
"out":[
{
"alias":"",
"projection":"t1.a"
},
{
"alias":"",
"projection":"t1.b"
}
],
"subselects":[],
"inputs":[
{
"executor":"TableScan",
"db":"db",
"table":"t1",
"alias":"t1",
"index":"PRIMARY KEY ()",
"storage":"lf_skiplist",
"dir":"forward",
"table_type":"sharded_rowstore",
"inputs":[]
}
]
}
]
}
],
"version":"4",
"info":{
"memsql_version":"7.1.2",
"memsql_version_hash":"5ae405fc1fe7cf3b9fc8cab1115e8080e67f2476",
"num_online_leaves":"1",
"num_online_aggs":"1",
"context_database":"db"
},
"query_info":{
"query_text":"EXPLAIN JSON INTO OUTFILE '\/tmp\/t1.json' SELECT * FROM t1",
"text_explain":"Gather partitions:all alias:remote_0\nProject [t1.a, t1.b]\nTableScan db.t1 table_type:sharded_rowstore\n"
}
Related Topics
- SingleStore DB Studio Visual Explain
- Query Plan Operations describes the components of a query plan.
- PROFILE displays detailed execution statistics in addition to execution operations.
