A query plan comprises a tree of nodes of different types. Each node may have one or more child nodes. Different nodes stand for different operations, but each parent node retrieves data from its child node row
The join type.For descriptions of the different types, see EXPLAIN Join Types. --对表所使用的访问方式。 2.1.5 possible_keys Thepossible_keys column indicates which indexes MySQL can choose from use to findthe rows in this table. Note that this column is totally independent of theorder of th...
shuffle data xxx in different shards! max rows is 20, min rows is 0: Data screw may exist. Note The advice part provides only suggestions for an SQL statement and may not be applicable. You need to perform optimization measures based on business scenarios. Cost The cost part provides ...
Supposethat MIN(c1) can be retrieved by reading one index row and MIN(c2) canbe retrieved by reading one row from a different index. That is, for eachcolumn c1 and c2, thereexists an index where the column is the first column of the index. In thiscase, one row is returned, produced...
Part of[EPIC] CompleteSQL EXPLAINTree Rendering#14914 @irenjjadded a newtreeexplain mode in#14677. Now we need to add support for different types of operators. Here is an example of how to see the new explain plans: setdatafusion.explain.format='tree';createtablefoo(xint, yint)asvalues(1...
the subquery is re-evaluated only once for each set of differentvalues of the variables from its outer context. For UNCACHEABLE SUBQUERY, thesubquery is re-evaluated for each row of the outer context. Cacheability ofsubqueries is subject to the restrictions detailed in Section 7.9.3.1, “How...
follow when creating adatabasefor use with an application‚ addressing the basic tasks encountered by adatabaseadministrator‚ and identifying and discussing some of the aspects of ongoingdatabasemanagement. In Week One‚ you are introduced to a variety ofdifferentdatabasearchitecturetyp...
Different from EXPLAIN, EXPLAIN ANALYZE executes the corresponding SQL statement, records its runtime information, and returns the information together with the execution plan. Therefore, you can regard EXPLAIN ANALYZE as an extension of the EXPLAIN statement. Compared to EXPLAIN (...
Transfers data between worker nodes for different stages of a query. The value for exchange_type can be one of the logical or distributed exchange types as described later in this section. Logical Exchange types The following exchange types describe actions taken during the exchange phase of a lo...
Under certain conditions the plan shown when using EXPLAIN PLAN can be different from the plan shown using V$SQL_PLAN. For example, when the SQL statement contains bind variables the plan shown from using EXPLAIN PLAN ignores the bind variable values while the plan shown in V$SQL_PLAN takes ...