Re: How can I manually alter the statistics for a column?

On Mon, Jun 1, 2009 at 8:20 PM, Douglas Alan <darkwater42(at)gmail(dot)com> wrote:
> I'd like to manually alter the statistics for a column, as for the column in
> question the statistics are causing Postgres to do the wrong thing for my
> purposes. (I.e., a Seq Scan, rather than an Index Scan.)  If someone can
> tell me how to achieve this, I would quite grateful.
>

Assuming estimates are off, wouldn't increasing the stats target be a
better approach? Then adjust the planner cost constants, in particular
seq_page_cost, random_page_cost and effective_cache_size, to prevent a
seq scan?

Considering you've pulled off the workaround, you must know what
you're doing, but I'd be worried about it backfiring in the future.

Regards,
Isak

> Thanks!
> |>ouglas
>
>
> P.S. Actually, for this particular problem, just deleting the statistics
> would be fine.  I've tried doing:
>
> alter table maindb_astobject alter column survey_id set statistics 0;
>
> And then analyzing the column, but when "statistics"  for a column are set
> to 0, Postgres seems to leave the current statistics in place, which is not
> the right thing for me at all.  I can successfully set "statistics" to 1,
> but that turns out to be one statistic too many.
>
> I've tried settings the statistics via the table "pg_stats", but that turns
> out to be a view, and Postgres won't allow to me to alter it.
>
> Perhaps I can achieve the end by altering the "pg_statistic" table instead,
> but that table is more than a bit opaque to me.
>
> P.P.S The Seq Scan is 2-4 orders of magnitude slower than the Index Scan.
>
>
>
>