Re: postgres 7.4 vs 8.x redux: query plans

On 4/3/07, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> "Alex Deucher" <alexdeucher(at)gmail(dot)com> writes:
> > Turning off bitmapscan ends up doing a sequential scan. Turning off
> > both bitmapscan and seqscan results in a bitmap heap scan. It doesn't
> > seem to want to use the index at all. Any ideas?
>
> The "ORed indexscans" plan style that was in 7.4 isn't there anymore;
> we use bitmap OR'ing instead. There actually are repeated indexscans
> hidden under the "= ANY" indexscan condition in 8.2, it's just that
> the mechanism for detecting duplicate matches is different. AFAIK the
> index access costs ought to be about the same either way, and the other
> costs the same or better as what we did in 7.4. It's clear though that
> 8.2 is taking some kind of big hit in the index access in your case.
> There's something very strange going on here.
>
> You do have both lc_collate and lc_ctype set to C, right? What about
> database encoding?

SHOW lc_collate ;
lc_collate
------------
C
(1 row)

SHOW lc_ctype ;
lc_ctype
----------
C
(1 row)

The encoding is UTF8, however I also built a SQL_ASCII version of the
DB to compare performance, but they both seem to perform about the
same.

Alex

SQL_ASCII:

EXPLAIN ANALYZE select num, c1, c2, c3, c4, c5, c6, c7, c8, c9, c10,
c11 from t1 where num in
('AB5679927','AB4974075','AB5066236','AB4598969','AB5009616','AB6409547','AB5593311','AB4975084','AB6604964','AB5637015','AB5135405','AB4501459','AB5605469','AB5603634','AB6000955','AB5718599','AB5328380','AB4846727');

QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on t1 (cost=80.41..152.72 rows=18 width=157)
(actual time=157.210..283.140 rows=18 loops=1)
Recheck Cond: ((num)::text = ANY
(('{AB5679927,AB4974075,AB5066236,AB4598969,AB5009616,AB6409547,AB5593311,AB4975084,AB6604964,AB5637015,AB5135405,AB4501459,AB5605469,AB5603634,AB6000955,AB5718599,AB5328380,AB4846727}'::character
varying[])::text[]))
-> Bitmap Index Scan on t1_pkey (cost=0.00..80.41 rows=18
width=0) (actual time=140.419..140.419 rows=18 loops=1)
Index Cond: ((num)::text = ANY
(('{AB5679927,AB4974075,AB5066236,AB4598969,AB5009616,AB6409547,AB5593311,AB4975084,AB6604964,AB5637015,AB5135405,AB4501459,AB5605469,AB5603634,AB6000955,AB5718599,AB5328380,AB4846727}'::character
varying[])::text[]))
Total runtime: 283.214 ms
(5 rows)

UTF8:

EXPLAIN ANALYZE select num, c1, c2, c3, c4, c5, c6, c7, c8, c9, c10,
c11 from t1 where num in
('AB5679927','AB4974075','AB5066236','AB4598969','AB5009616','AB6409547','AB5593311','AB4975084','AB6604964','AB5637015','AB5135405','AB4501459','AB5605469','AB5603634','AB6000955','AB5718599','AB5328380','AB4846727');

QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on t1 (cost=80.41..152.72 rows=18 width=159)
(actual time=126.194..126.559 rows=18 loops=1)
Recheck Cond: ((num)::text = ANY
(('{AB5679927,AB4974075,AB5066236,AB4598969,AB5009616,AB6409547,AB5593311,AB4975084,AB6604964,AB5637015,AB5135405,AB4501459,AB5605469,AB5603634,AB6000955,AB5718599,AB5328380,AB4846727}'::character
varying[])::text[]))
-> Bitmap Index Scan on t1_pkey (cost=0.00..80.41 rows=18
width=0) (actual time=126.155..126.155 rows=18 loops=1)
Index Cond: ((num)::text = ANY
(('{AB5679927,AB4974075,AB5066236,AB4598969,AB5009616,AB6409547,AB5593311,AB4975084,AB6604964,AB5637015,AB5135405,AB4501459,AB5605469,AB5603634,AB6000955,AB5718599,AB5328380,AB4846727}'::character
varying[])::text[]))
Total runtime: 126.661 ms
(5 rows)