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EXPLAIN — show the execution plan of a statement.
Syntax
EXPLAIN [ ( option [, ...] ) ] statementwhere option can be one of:ANALYZE [ boolean ]VERBOSE [ boolean ]FORMAT { TEXT | JSON }
Description
This command displays the execution plan that the MatrixOne planner generates for the supplied statement. The execution plan shows how the table(s) referenced by the statement will be scanned — by plain sequential scan, index scan, and so on. — 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 (measured in cost units that are arbitrary, but conventionally mean disk page fetches). Actually two numbers are shown: the start-up cost before the first row can be returned, and the total cost to return all the rows. For most queries the total cost is what matters, but in contexts such as a subquery in EXISTS, the planner will choose the smallest start-up cost instead of the smallest total cost (since the executor will stop after getting one row, anyway). Also, if you limit the number of rows to return with a LIMIT clause, the planner makes an appropriate interpolation between the endpoint costs to estimate which plan is really the cheapest.
The ANALYZE option causes the statement to be actually executed, not only planned. Then actual run time 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 for seeing whether the planner’s estimates are close to reality.
Parameters
- ANALYZE:
Carry out the command and show actual run times and other statistics. This parameter defaults to FALSE.
- VERBOSE:
Display additional information regarding the plan. Specifically, include the output column list for each node in the plan tree, schema-qualify table and function names, always label variables in expressions with their range table alias, and always print the name of each trigger for which statistics are displayed. This parameter is FALSE by default.
- FORMAT:
Specify the output format, which can be TEXT, JSON. Non-text output contains the same information as the text output format, but is easier for programs to parse. This parameter is TEXT by dafault.
- BOOLEAN:
Specifies whether the selected option should be turned on or off. You can write TRUEto enable the option, and FALSE to disable it. The boolean value can also be omitted, in which case TRUE is assumed.
- STETEMENT
MatrixOne supports any SELECT, UPDATE, DELETE statement execution plan. For INSERT statement, only INSERT INTO..SELECT is supported in 0.5.1 version. INSERT INTO...VALUES is not supported yet.
Examples
Node_TABLE_SCAN
mysql> explain verbose SELECT N_NAME, N_REGIONKEY a FROM NATION WHERE N_NATIONKEY > 0 OR N_NATIONKEY < 10;+------------------------------------------------------------------------------------+| QUERY PLAN |+------------------------------------------------------------------------------------+| Project(cost=0.00..0.00 card=25.00 ndv=0.00 rowsize=0) || Output: #[0,0], #[0,1] || -> Table Scan on db1.nation(cost=0.00..0.00 card=25.00 ndv=0.00 rowsize=0) || Output: #[0,1], #[0,2] || Table: 'nation' (0:'n_nationkey', 1:'n_name', 2:'n_regionkey') || Filter Cond: ((CAST(#[0,0] AS INT64) > 0) or (CAST(#[0,0] AS INT64) < 10)) |+------------------------------------------------------------------------------------+
Node_VALUE_SCAN
mysql> explain verbose select abs(-1);+-----------------------------------------------------------------------------+| QUERY PLAN |+-----------------------------------------------------------------------------+| Project(cost=0.00..0.00 card=1.00 ndv=0.00 rowsize=0) || Output: 1 || -> Values Scan "*VALUES*" (cost=0.00..0.00 card=1.00 ndv=0.00 rowsize=0) || Output: 0 |+-----------------------------------------------------------------------------+
Node_SORT
mysql> explain verbose SELECT N_NAME, N_REGIONKEY a FROM NATION WHERE N_NATIONKEY > 0 AND N_NATIONKEY < 10 ORDER BY N_NAME, N_REGIONKEY DESC;+--------------------------------------------------------------------------------------------+| QUERY PLAN |+--------------------------------------------------------------------------------------------+| Project(cost=0.00..0.00 card=25.00 ndv=0.00 rowsize=0) || Output: #[0,0], #[0,1] || -> Sort(cost=0.00..0.00 card=25.00 ndv=0.00 rowsize=0) || Output: #[0,0], #[0,1] || Sort Key: #[0,0] INTERNAL, #[0,1] DESC || -> Project(cost=0.00..0.00 card=25.00 ndv=0.00 rowsize=0) || Output: #[0,0], #[0,1] || -> Table Scan on db1.nation(cost=0.00..0.00 card=25.00 ndv=0.00 rowsize=0) || Output: #[0,1], #[0,2] || Table: 'nation' (0:'n_nationkey', 1:'n_name', 2:'n_regionkey') || Filter Cond: (CAST(#[0,0] AS INT64) > 0), (CAST(#[0,0] AS INT64) < 10) |+--------------------------------------------------------------------------------------------+
With limit and offset:
mysql> explain SELECT N_NAME, N_REGIONKEY FROM NATION WHERE abs(N_REGIONKEY) > 0 AND N_NAME LIKE '%AA' ORDER BY N_NAME DESC, N_REGIONKEY limit 10;+-------------------------------------------------------------------------------------------+| QUERY PLAN |+-------------------------------------------------------------------------------------------+| Project(cost=0.00..0.00 card=25.00 ndv=0.00 rowsize=0) || -> Sort(cost=0.00..0.00 card=25.00 ndv=0.00 rowsize=0) || Sort Key: #[0,0] DESC, #[0,1] INTERNAL || Limit: 10 || -> Project(cost=0.00..0.00 card=25.00 ndv=0.00 rowsize=0) || -> Table Scan on db1.nation(cost=0.00..0.00 card=25.00 ndv=0.00 rowsize=0) || Filter Cond: (abs(CAST(#[0,1] AS INT64)) > 0), (#[0,0] like '%AA') |+-------------------------------------------------------------------------------------------+
mysql> explain SELECT N_NAME, N_REGIONKEY FROM NATION WHERE abs(N_REGIONKEY) > 0 AND N_NAME LIKE '%AA' ORDER BY N_NAME DESC, N_REGIONKEY LIMIT 10 offset 20;+-------------------------------------------------------------------------------------------+| QUERY PLAN |+-------------------------------------------------------------------------------------------+| Project(cost=0.00..0.00 card=25.00 ndv=0.00 rowsize=0) || -> Sort(cost=0.00..0.00 card=25.00 ndv=0.00 rowsize=0) || Sort Key: #[0,0] DESC, #[0,1] INTERNAL || Limit: 10, Offset: 20 || -> Project(cost=0.00..0.00 card=25.00 ndv=0.00 rowsize=0) || -> Table Scan on db1.nation(cost=0.00..0.00 card=25.00 ndv=0.00 rowsize=0) || Filter Cond: (abs(CAST(#[0,1] AS INT64)) > 0), (#[0,0] like '%AA') |+-------------------------------------------------------------------------------------------+
Node_AGG
mysql> explain verbose SELECT count(*) FROM NATION group by N_NAME;+-------------------------------------------------------------------------------------+| QUERY PLAN |+-------------------------------------------------------------------------------------+| Project(cost=0.00..0.00 card=25.00 ndv=0.00 rowsize=0) || Output: #[0,0] || -> Aggregate(cost=0.00..0.00 card=25.00 ndv=0.00 rowsize=0) || Output: #[-2,0] || Group Key:#[0,1] || Aggregate Functions: starcount(#[0,0]) || -> Table Scan on db1.nation(cost=0.00..0.00 card=25.00 ndv=0.00 rowsize=0) || Output: #[0,0], #[0,1] || Table: 'nation' (0:'n_nationkey', 1:'n_name') |+-------------------------------------------------------------------------------------+
Node_JOIN
mysql> explain verbose SELECT NATION.N_NAME, REGION.R_NAME FROM NATION join REGION on NATION.N_REGIONKEY = REGION.R_REGIONKEY WHERE NATION.N_REGIONKEY > 10 AND LENGTH(NATION.N_NAME) > LENGTH(REGION.R_NAME);+--------------------------------------------------------------------------------------------+| QUERY PLAN |+--------------------------------------------------------------------------------------------+| Project(cost=0.00..0.00 card=125.00 ndv=0.00 rowsize=0) || Output: #[0,1], #[0,0] || -> Filter(cost=0.00..0.00 card=125.00 ndv=0.00 rowsize=0) || Output: #[0,0], #[0,1] || Filter Cond: (length(CAST(#[0,1] AS CHAR)) > length(CAST(#[0,0] AS CHAR))) || -> Join(cost=0.00..0.00 card=125.00 ndv=0.00 rowsize=0) || Output: #[0,1], #[1,0] || Join Type: INNER || Join Cond: (#[1,1] = #[0,0]) || -> Table Scan on tpch.region(cost=0.00..0.00 card=5.00 ndv=0.00 rowsize=0) || Output: #[0,0], #[0,1] || Table: 'region' (0:'r_regionkey', 1:'r_name') || -> Table Scan on tpch.nation(cost=0.00..0.00 card=25.00 ndv=0.00 rowsize=0) || Output: #[0,0], #[0,1] || Table: 'nation' (0:'n_name', 1:'n_regionkey') || Filter Cond: (CAST(#[0,1] AS INT64) > 10) |+--------------------------------------------------------------------------------------------+
Node_INSERT
mysql> explain verbose INSERT NATION select * from nation;+---------------------------------------------------------------------------------------------+| QUERY PLAN |+---------------------------------------------------------------------------------------------+| Insert on db1.nation (cost=0.0..0.0 rows=0 ndv=0 rowsize=0) || Output: #[0,0], #[0,1], #[0,2], #[0,3] || -> Project(cost=0.00..0.00 card=25.00 ndv=0.00 rowsize=0) || Output: #[0,0], #[0,1], #[0,2], #[0,3] || -> Table Scan on db1.nation(cost=0.00..0.00 card=25.00 ndv=0.00 rowsize=0) || Output: #[0,0], #[0,1], #[0,2], #[0,3] || Table: 'nation' (0:'n_nationkey', 1:'n_name', 2:'n_regionkey', 3:'n_comment') |+---------------------------------------------------------------------------------------------+7 rows in set (0.00 sec)
Node_Update
mysql> explain verbose UPDATE NATION SET N_NAME ='U1', N_REGIONKEY=2 WHERE N_NATIONKEY > 10 LIMIT 20;+-------------------------------------------------------------------------------------+| QUERY PLAN |+-------------------------------------------------------------------------------------+| Update on db1.nation (cost=0.0..0.0 rows=0 ndv=0 rowsize=0) || -> Project(cost=0.00..0.00 card=25.00 ndv=0.00 rowsize=0) || Output: #[0,0], 'U1', CAST(2 AS INT32) || Limit: 20 || -> Table Scan on db1.nation(cost=0.00..0.00 card=25.00 ndv=0.00 rowsize=0) || Output: #[0,1] || Table: 'nation' (0:'n_nationkey', 1:'PADDR') || Filter Cond: (CAST(#[0,0] AS INT64) > 10) |+-------------------------------------------------------------------------------------+
Node_Delete
mysql> explain verbose DELETE FROM NATION WHERE N_NATIONKEY > 10;+-------------------------------------------------------------------------------------+| QUERY PLAN |+-------------------------------------------------------------------------------------+| Delete on db1.nation (cost=0.0..0.0 rows=0 ndv=0 rowsize=0) || -> Project(cost=0.00..0.00 card=25.00 ndv=0.00 rowsize=0) || Output: #[0,0] || -> Table Scan on db1.nation(cost=0.00..0.00 card=25.00 ndv=0.00 rowsize=0) || Output: #[0,1] || Table: 'nation' (0:'n_nationkey', 1:'PADDR') || Filter Cond: (CAST(#[0,0] AS INT64) > 10) |+-------------------------------------------------------------------------------------+
With limit:
mysql> explain verbose DELETE FROM NATION WHERE N_NATIONKEY > 10 LIMIT 20;+-------------------------------------------------------------------------------------+| QUERY PLAN |+-------------------------------------------------------------------------------------+| Delete on db1.nation (cost=0.0..0.0 rows=0 ndv=0 rowsize=0) || -> Project(cost=0.00..0.00 card=25.00 ndv=0.00 rowsize=0) || Output: #[0,0] || Limit: 20 || -> Table Scan on db1.nation(cost=0.00..0.00 card=25.00 ndv=0.00 rowsize=0) || Output: #[0,1] || Table: 'nation' (0:'n_nationkey', 1:'PADDR') || Filter Cond: (CAST(#[0,0] AS INT64) > 10) |+-------------------------------------------------------------------------------------+
