![]() ' Using where Using temporary Using filesort' in the Without the FORCE INDEX option we would have WHERE population >= 10000000 GROUP BY Name This is used after the table name in the FROM clause.Įxample: CREATE INDEX Name ON City (Name) You can limit which indexes are considered with the USE INDEX Limited set of indexes, ignore one or more indexes, or force the usage of some In these cases you have the option to either tell the optimizer to only use a Not use an index at all, even if some index could theoretically be used. In some cases the optimizer may choose a non-optimal index or it may choose to Forcing Usage of a Specific Index for the WHERE Clause This is one of the few cases where ALL is ok, as the scan of theĬountry table will only find one matching row. Using where Using join buffer (flat, BNL join) The output of EXPLAIN for the above cases is: As there is only one matching country this will be faster than the Matching country (one in this case) all rows in City will be checked for a In both of the above cases Country will be scanned first and for each For example: SELECT SUM(City.Population) FROM Country STRAIGHT_JOIN City WHERE When this is done, only tablesĬonnected with STRAIGHT_JOIN will have their order forced. ![]() If you only want to force the join order for a few tables, use Order in the FROM clause and use SELECT STRAIGHT_JOIN like so: SELECT STRAIGHT_JOIN SUM(City.Population) FROM Country,City WHEREĬity.Countr圜ode=Country.Code AND Country.HeadOfState="Volodymyr Zelenskyy" The simplest way to force the join order is to put the tables in the correct You can force the join order by using STRAIGHT_JOIN either in In some cases, its output can be closer to reality than EXPLAIN.įor the following queries, we will use the world database for SHOW EXPLAIN shows the output of a running query. You can examine the query plan for a SELECT by writingĮXPLAIN before the statement. However in some cases it does not have enough information to chooseĪ perfect plan and in these cases you may have to provide hints to force the The optimizer is largely cost-based and will try to choose the optimal plan forĪny query. Forcing/Disallowing TemporaryTables to be Used for GROUP BY:.Help the Optimizer Optimize GROUP BY and ORDER BY.Forcing an Index to be Used for ORDER BY or GROUP BY.IGNORE INDEX: Don't Use a Particular Index.USE INDEX: Use a Limited Set of Indexes. ![]()
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