Re: seqscan for 100 out of 3M rows, index present

From: Willy-Bas Loos [mailto:willybas@gmail.com]
Sent: Wednesday, June 26, 2013 3:19 PM
To: Igor Neyman
Cc: pgsql-performance@postgresql.org
Subject: Re: [PERFORM] seqscan for 100 out of 3M rows, index present

plan with enable_seqscan off:

Aggregate  (cost=253892.48..253892.49 rows=1 width=0) (actual time=208.681..208.681 rows=1 loops=1)
  ->  Nested Loop  (cost=5.87..253889.49 rows=1198 width=0) (actual time=69.403..208.647 rows=17 loops=1)
        ->  Index Scan using geo_blok_idx on geo g  (cost=0.00..1314.43 rows=500 width=8) (actual time=45.776..46.147
rows=121loops=1) 
              Index Cond: (blok = 1942)
        ->  Bitmap Heap Scan on bmp_data d  (cost=5.87..502.91 rows=179 width=8) (actual time=1.340..1.341 rows=0
loops=121)
              Recheck Cond: (geo_id = g.geo_id)
              ->  Bitmap Index Scan on bmp_data_geo_idx  (cost=0.00..5.82 rows=179 width=0) (actual time=1.206..1.206
rows=0loops=121) 
                    Index Cond: (geo_id = g.geo_id)
Total runtime: 208.850 ms


On Wed, Jun 26, 2013 at 9:08 PM, Igor Neyman <ineyman@perceptron.com> wrote:


From: Willy-Bas Loos [mailto:willybas@gmail.com]
Sent: Wednesday, June 26, 2013 3:04 PM
To: Igor Neyman
Cc: pgsql-performance@postgresql.org
Subject: Re: [PERFORM] seqscan for 100 out of 3M rows, index present

nope
$ grep ^[^#] /etc/postgresql/9.1/main/postgresql.conf|grep -e ^[^[:space:]]
data_directory = '/var/lib/postgresql/9.1/main'        # use data in another directory
hba_file = '/etc/postgresql/9.1/main/pg_hba.conf'    # host-based authentication file
ident_file = '/etc/postgresql/9.1/main/pg_ident.conf'    # ident configuration file
external_pid_file = '/var/run/postgresql/9.1-main.pid'        # write an extra PID file
port = 5432                # (change requires restart)
max_connections = 100            # (change requires restart)
unix_socket_directory = '/var/run/postgresql'        # (change requires restart)
ssl = true                # (change requires restart)
shared_buffers = 2GB            # min 128kB
work_mem = 100MB                # min 64kB
maintenance_work_mem = 256MB        # min 1MB
synchronous_commit = off        # synchronization level; on, off, or local
checkpoint_segments = 10        # in logfile segments, min 1, 16MB each
log_line_prefix = '%t '            # special values:
datestyle = 'iso, mdy'
lc_messages = 'en_US.UTF-8'            # locale for system error message
lc_monetary = 'en_US.UTF-8'            # locale for monetary formatting
lc_numeric = 'en_US.UTF-8'            # locale for number formatting
lc_time = 'en_US.UTF-8'                # locale for time formatting
default_text_search_config = 'pg_catalog.english'
--

How much RAM you have on this machine?
What else is this machine is being used for (besides being db server)?
And, what is your setting for effective_cache_size?  It looks like you didn't change it from default (128MB).
You need to adjust effective_cache_size so somewhat between 60%-75% of RAM, if the database is the main process running
onthis machine. 

Again, effective_cache_size could be set on session level, so you could try it before changing GUC in postgresql.conf.
When trying it, don't forget to change enable_seqscan back to "on" (if it's still "off").

Igor Neyman