![]() ![]() Rewrite the WHERE clauses to eliminate dead code, reducing the unnecessary computations and changing the constraints wherever possible to the open the way for using keys.Determine the order in which tables should be joined when more than one table is present in the query.If there are a lot of records that match the key value, the advantages of the key are reduced and the table scan becomes faster. For each table, decide whether a table scan is better that reading on a key.Determine which keys can be used to retrieve the records from tables, and choose the best one for each table.Why ? Two reasons: REASON #1 : Evaluation Strategyīy design, queries are evaluated in a loop using the following algorithm: There are also some optimizer switches that affect which algorithms will be considered when optimizing a query. Block Index Hash join known as Batch Key Access Hash (BKAH) join.Block Index join known as Batch Key Access (BKA) join.This page documents the various block-based join algorithms. ![]() Block-based join algorithms in MariaDB employ a join buffer to accumulate records of the first join operand before they start looking for matches in the second join operand. MariaDB 5.3 (and later) enhances the implementation of BNL joins and provides a variety of block-based join algorithms that can be used for inner joins, outer joins, and semi-joins. In the versions of MariaDB/MySQL before 5.3 only one block-based join algorithm was implemented: the Block Nested Loops (BNL) join algorithm. In their Block-Based Join Algorithms page, they describe the new methods: MariaDB, a MySQL fork has implemented various other join methods in the latest (5.3 and 5.5) releases which can be drop-in replacements of MySQL (5.1 and 5.5 respectively) versions. In other words, no hash join algorith or variation has been implemented, not even in version 5.7 - although that is still in development (hope never dies). MySQL resolves all joins using a nested-loop join method. Unfortunately, the only method that the optimizer knows is nested-loop method. Up to the (recently released) 5.6 version, no, you can't.
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