也就是说,对于t1中的每一行,MySQL只需要在t2中只执行一次查找,而不考虑在t2中实际匹配的行数。 在MySQL 8.0.17及更高版本中,如果出现此提示,还可表示形如 NOT IN (subquery) 或 NOT EXISTS (subquery) 的WHERE条件已经在内部转换为反连接。这将删除子查询并将其表放入最顶层的查询计划中,从而改进查询的开销...
This tableis referenced as the child of table in a join that can be pushed down to the NDBkernel. Applies only in MySQL Cluster, when pushed-down joins are enabled. Seethe description of the ndb_join_pushdown serversystem variable for more information and examples. const row not found (JSON...
EXPLAIN returnsa row of information for each table used in the SELECT statement. It lists thetables in the output in the order that MySQL would read them while processingthe statement. MySQL resolves all joins using a nested-loop join method. Thismeans that MySQL reads a row from the first ...
SQL - IS NULL SQL - IS NOT NULL SQL - NOT NULL SQL - BETWEEN Operator SQL - UNION Operator SQL - UNION vs UNION ALL SQL - INTERSECT Operator SQL - EXCEPT Operator SQL - Aliases SQL Joins SQL - Using Joins SQL - Inner Join SQL - Left Join SQL - Right Join SQL - Cross Join SQL...
It makes these decisions based on the statistical information it has about your data and by leveraging Oracle database features such as hash joins, parallel query, partitioning, etc. Still it is expected that the optimizer will generate sub-optimal plans for some SQL statements now and then. ...
If SQL query performance is poor or query results do not meet expectations, you can execute the EXPLAIN and EXPLAIN ANALYZE statements in Hologres to obtain execution plans of query statements. This way, you can optimize query statements or database structures. This topic describes how to execute...
With the help ofEXPLAIN, you can see where you should add indexes to tables so that thestatement executes faster by using indexes to find rows. You can also useEXPLAIN to check whether the optimizer joins the tables in an optimal order. Togive a hint to the optimizer to use a join orde...
EXPLAIN returnsa row of information for each table used in the SELECT statement. It lists thetables in the output in the order that MySQL would read them while processingthe statement. MySQL resolves all joins using a nested-loop join method. Thismeans that MySQL reads a row from the first...
Actual Time:The real time taken in milliseconds. Rows:Number of rows processed at each node level. Loops:Times a node is executed (especially useful for understanding nested loops). Query Optimization Insights: If the EXPLAIN ANALYZE output shows high execution time or slow node types (e.g.,...
Joins data between two tables. Joins can be categorized by join type and by distribution type. Join types Join types define the way in which the join operation occurs. CrossJoin –Produces the Cartesian product of the two tables joined. InnerJoin –Selects records that have matching values in...