Re: index not being pick up

[Peter Brawley]

Given tables a, b with matchable indexed id columns, Explain will tell you that the query engine will not use an index on table a in the query ...

select a.* from a join b using(id);

... because it sees that the query requires the engine to read all the data in table a, so an index on it won't help performance.

But it will use an index for ...

select b.* from a join b using(id);

... because now the index offers an advantage.