8/17/2023 0 Comments Mysql optimizer hintWith every DDL, assume every hint applied to that object is going to go wrong.With every Oracle patch or upgrade, assume every hint is going to go wrong.If you must use hints, assume you’ve used them incorrectly.So why am I painting optimizer hints as the baddies here? My All Things Oracle colleague Jonathan Lewis tells us why in his Rules for Hinting (imagine Brad Pitt reading these out, if you like: The first rule of Fight Club is…): Hints are injected into DML statements within the bounds of a comment. In fact, the very name is a misnomer – they are not hints they are commands that override the optimizer (as long as the hint is valid and the _OPTIMIZER_IGNORE_HINTS initialization parameter is not TRUE). Hints give us the opportunity, in cases where we have superior knowledge about the database, to influence the optimizer. An optimizer hint is a code snippet within an SQL statement controlling the decisions of the optimizer. The optimizer is the hero of our story let me introduce the potential villains, Optimizer Hints. (You can study the explain plan to see what decisions the optimizer has taken.) The optimizer arrives at its conclusions, often in barely a whisper of time.Īnd when the SQL statement is executed, you sit back and you feel like a genius.Īnd that’s it, right? THE END. Your database automatically gathers stats about your objects – stuff like the number of rows, number of distinct values, of nulls, data distribution – and the optimizer uses this information in its decision-making. The optimizer analyses your SQL statement and decides the most efficient way to execute it based on the objects involved in the statement and the conditions you’re subjecting them to. A number of engines silently spring into action including the optimizer. It is often easy to forget this, but in many ways it is after we hit the execute button that the really exciting stuff starts with our code.
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