Re: n_distinct off by a factor of 1000

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 think you are counting 2 different things here.

The first query returns all the columns "like 'instr%_ref'" present in the statistics (so in the whole cluster), while the second is counting the actual number of different rows in bigtable.


regards,

fabio pardi