Re: AW: [HACKERS] create index updates nrows statistics

ZEUGSWETTER Andreas IZ5 <Andreas(dot)Zeugswetter(at)telecom(dot)at> writes:
>>>> a create index updates the statistics in pg_class,
>>>> this leads to substantial performance degradation compared to
>>>> 6.4.2.
>>
>> Create index did that in 6.4.2 as well --- how could it be making
>> performance worse?
>>
> I am not sure why, but in 6.4.2 a create table, create index, insert,
> select * from tab where indexedcol=5 did actually use the index path,
> even if table reltuples and relpages was 0.

Hmm, you're right. Using 6.4.2:

play=> create table foobar (f1 int4);
CREATE
play=> explain select * from foobar where f1 = 4;
NOTICE: QUERY PLAN:

Seq Scan on foobar (cost=0.00 size=0 width=4)

play=> create index foobar_f1 on foobar(f1);
CREATE
play=> explain select * from foobar where f1 = 4;
NOTICE: QUERY PLAN:

Index Scan using foobar_f1 on foobar (cost=0.00 size=0 width=4)

whereas in 6.5 you still get a sequential scan because it estimates the
cost of the index scan at 1.0 not 0.0. I think I'm to blame for this
behavior change: I remember twiddling costsize.c to provide more
realistic numbers for an index scan, and in particular to ensure that
an index scan would be considered more expensive than a sequential scan
unless it was able to eliminate a useful number of rows. But when
the estimated relation size is zero (or very small) the selectivity
benefit can't make up even a mere 1.0 cost bias.

I believe 6.5 is operating as it should --- 6.4 was producing inferior
plans for small tables. But it is clearly a Bad Thing to allow the 6.5
optimizer to believe that a relation is empty when it isn't. I concur
with your suggestion to hack up CREATE INDEX so that creating an index
before you load the table isn't quite such a losing proposition.

> Please apply the patch I previously sent.

Will do.

regards, tom lane