Re: BUG #4462: Adding COUNT to query causes massive slowdown

On Thu, Oct 9, 2008 at 3:05 PM, Peter Eisentraut <peter_e(at)gmx(dot)net> wrote:

> Jussi Pakkanen wrote:
>>
>> However when I try to count the amount of distinct codes, I get this:
>>
>>
>> EXPLAIN SELECT COUNT(DISTINCT code) FROM log;
>> QUERY PLAN
>>
>> ----------------------------------------------------------------------------
>> -----
>> Aggregate (cost=100801488.30..100801488.31 rows=1 width=10)
>> -> Seq Scan on log (cost=100000000.00..100721245.24 rows=32097224
>> width=10)
>> (2 rows)
>
> This looks like you have one of the enable_${plantype} parameters turned
> off. 100000000 is the penalty that is added when a plantype if turned off.

This was caused by enable_seqscan. When I set it to 'on', the penalty
disappears but it still does the full table scan.

Given that PostgreSQL does the scan even with the huge seqscan
penalty, I can think of only two different causes:

1) some sort of a bug in the query analyzer
2) SELECT COUNT(DISTINCT x) for some reason requires information that
is not available in the index. The only one I could think of would be
NULL values, but the 'code' field is defined as 'NOT NULL'.

Also I had a small error in my original but report. There are 2
million distinct values of 'code', rather than 200 000.