Re: n_distinct off by a factor of 1000

Hello,

> I got my first hint of why this problem occurs when I looked at the
> statistics.  For the column in question, "instrument_ref" the
> statistics claimed it to be:
>
> The default_statistics_target=500, and analyze has been run.
> select * from pg_stats where attname like 'instr%_ref'; -- Result:
> *40.000*
> select count(distinct instrumentid_ref) from bigtable -- Result: *33
> 385 922 (!!)*
>
> That is an astonishing difference of almost a 1000X.

> I have tried to increase the statistics target to 5000, and it helps,
> but it reduces the error to 100X.  Still crazy high.

As far as I know, increasing default_statistics_target will not help. [1]

> I have considered these fixes:
> - hardcode the statistics to a particular ratio of the total number of
> rows

You can hardcode the percentage of distinct values:
ALTER TABLE bigtable ALTER COLUMN instrument_ref SET ( n_distinct=-0.06
); /* -1 * (33385922 / 500000000) */

[1]
https://www.postgresql.org/message-id/4136ffa0812111823u645b6ec9wdca60b3da4b00499%40mail.gmail.com

-----
Pavel Luzanov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company