Re: SELECT * FROM <table> LIMIT 1; is really slow

Tom Lane wrote:
>>INFO: "csn_edges": found 0 removable, 16289929 nonremovable row
>>versions in 2783986 pages
>
>
> That works out to just under 6 rows per 8K page, which wouldn't be too
> bad if the rows are 1K wide on average, but are they? (You might want
> to run contrib/pgstattuple to get some exact information about average
> tuple size.)

The rows are "wide" - there's a PostGIS geometry present.

>>INFO: analyzing "public.csn_edges"
>>INFO: "csn_edges": 2783986 pages, 3000 rows sampled, 6724 estimated
>>total rows
>
>
> This looks like a smoking gun to me. The huge underestimate of number
> of rows from ANALYZE is a known failure mode of the existing sampling
> method when the early pages of the table are thinly populated. (Manfred
> just fixed that for 7.5, btw.)
>
> I think you want to VACUUM FULL or CLUSTER the table, and then take a
> look at your FSM settings and routine vacuuming frequency to see if
> you need to adjust them to keep this from happening again.

I'm now clustering - thanks for the help!

The history of this table is quite short - I just created it last week.

The original table had a bigint column that I converted to int (using
the "alter table csn_edges rename to csn_edges_backup;CREATE TABLE
csn_edges AS SELECT a,b,c::int,d,e FROM csn_edges; delete table
csn_edges_backup;" trick). I dont think there were any changes to the
current csn_edges table after it was created.

I have another copy of this table in another database - vacuum analyse
verbose says its "only" 1,500,000 pages (vs 2,800,000). Shouldnt vacuum
know your table is wasting 10Gb of space and fix it for you? Or at
least HINT? Or a "TIDY" command?

Should I be upping my FSM to 2,000,000 pages?

dave