Re: query planner not using the correct index

Yeah with default_statistics_target at 500 most_common_vals had 4
values with the fourth having a frequency of 1.5% and distinct have
250+ in it.

How do I increase the stats target for just one column?

On Thu, Aug 7, 2008 at 6:48 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> "Joshua Shanks" <jjshanks(at)gmail(dot)com> writes:
>> SELECT null_frac, n_distinct, most_common_vals, most_common_freqs FROM
>> pg_stats WHERE tablename = 'bars' AND attname='bars_id';
>> null_frac | n_distinct | most_common_vals | most_common_freqs
>> -----------+------------+----------------------+---------------------------
>> 0 | 14 | {145823,47063,24895} | {0.484667,0.257333,0.242}
>
>> Those 3 values in reality and in the stats account for 98% of the
>> rows. actual distinct values are around 350
>
> So you need to increase the stats target for this column. With those
> numbers the planner is going to assume that any value that's not one
> of the big three appears about (1 - (0.484667+0.257333+0.242)) / 11
> of the time, or several hundred times in 300K rows. If n_distinct were
> up around 350 it would be estimating just a dozen or so occurrences,
> which should push the join plan into the shape you want. It's likely
> that it won't bother to include any more entries in most_common_vals
> no matter how much you raise the target; but a larger sample should
> definitely give it a better clue about n_distinct.
>
> regards, tom lane
>