Re: pg_trgm indexes giving bad estimations?

Now that I have a little time to work on this again, I've thought about it
and it seems that an easy and somewhat accurate cop-out to do this is to
use whatever the selectivity function would be for the like operator,
multiplied by a scalar that pg_tgrm should already have access to.

Unfortunately, it's not at all clear to me from reading
http://www.postgresql.org/docs/8.1/interactive/xoper-optimization.html#AEN33077
how like impliments selectivity. Any pointers on where to look?

On Wed, 4 Oct 2006, Tom Lane wrote:

> Ben <bench@silentmedia.com> writes:
>> How can I get the planner to not expect so many rows to be returned?
>
> Write an estimation function for the pg_trgm operator(s).  (Send in a
> patch if you do!)  I see that % is using "contsel" which is only a stub,
> and would likely be wrong for % even if it weren't.
>
>> A possibly related question is: because pg_tgrm lets me set the
>> matching threshold of the % operator, how does that affect the planner?
>
> It hasn't a clue about that.
>
>             regards, tom lane
>
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