Re: choosing use an index or not

On Thu, Dec 14, 2006 at 10:06:45AM -0300, Felipe Rondon Rocha wrote:

Hi everyone,

Do you know in which part of the file analyze.c, PostgreSQL decides
to use or not an index? I´m making a test and I can´t trace the
moment that he makes the choice.

It doesn't explicitly make the choice anywhere. The planner makes a set
of "paths" that can be used. So for each table a "path" is made for a
sequential scan and one or more "paths" for each index. For each a cost
is calcualted and whichever has the lowest cost is the one chosen.

It's quite dynamic, the choice also depends on what happens to the
result. There's no point using an index if the result is in the wrong
order for the join, for example.

Have a nice day,
--
Martijn van Oosterhout <kleptog@svana.org> http://svana.org/kleptog/