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Asked: 2026-06-16T16:55:45+00:00

Building on this answer: https://stackoverflow.com/a/2349824/1382306 Are there further guidelines in A and B’s usages,

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Building on this answer: https://stackoverflow.com/a/2349824/1382306

Are there further guidelines in A and B’s usages, or are they equally important?

I have a SELECT FROM table1, table2 WHERE table2.table1id = table1.id AND table2.constant = ?.

In my case, I know that ? is from a user-defined variable. Does that make a difference since it never changes?

Could I push my luck and ask for an explanation of the logic used against (A,B) and (B), like “all Bs are found that have As, and all Bs are searched next”.

Many thanks in advance!

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  1. Editorial Team

    Hidden in another popular post https://stackoverflow.com/a/4120191/1382306, it looks like

    “Queries need to address the order of columns when dealing with a covering index (an index on more than one column), from left to right in index column definition. The column order in the statement doesn’t matter, only that of columns 1, 2 and 3 – a statement needs have a reference to column 1 before the index can be used. If there’s only a reference to column 2 or 3, the covering index for 1/2/3 could not be used.

    In MySQL, only one index can be used per SELECT/statement in the query (subqueries/etc are seen as a separate statement). And there’s a limit to the amount of space per table that MySQL allows. Additionally, running a function on an indexed column renders the index useless”

    There’s a good list of index pitfalls in that answer too.

    In my case since “only one index can be used per SELECT/statement in the query” I’m guessing that using only (?, table2.table1id) is the way to go.

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