Thread: Using Bitmap scan instead of Seq scan

Greetings guys,

I am running PostgreSQL 8.4, my database is 60GB. My problem is that
there are some queries run extremely fast (when bitmap scan is used),
while others take more than 600secs to respond (when seq scan is used).

Here is a sample of the queries which uses seq scan...

[2011-02-14 09:15:45.464 PST] <schema1> 6828: LOG:  duration: 620577.665
ms  plan:
         Sort  (cost=1530070.15..1530072.12 rows=790 width=1539) (actual
time=620453.962..620458.985 rows=3404 loops=1)
           Sort Key: members.state, (ts_rank(members.ftsearch,
to_tsquery('personal & care & Barbers'::text)))
           Sort Method:  quicksort  Memory: 2103kB
           ->  Seq Scan on members  (cost=0.00..1530032.12 rows=790
width=1539) (actual time=47559.697..620422.036 rows=3404 loops=1)
                 Filter: (((edited_time IS NOT NULL) OR
((member_type)::text = 'paid_business'::text)) AND (ftsearch @-@
to_tsquery('personal & care & Barbers'::text)) AND (ts_rank(ftsearch,
to_tsquery('personal & care & Barbers'::text)) > 0.3::double precision))
[2011-02-14 09:15:45.464 PST] <schema1> 6828: STATEMENT:  SELECT * FROM
(SELECT ts_rank(ftsearch, to_tsquery('personal & care & Barbers')) as

this_rank,members.status,members.business_hours,members.keywords01,members.first_name,members.last_name,members.address01,members.city,members.state,members.address02,members.asn_member_id,members.keywords02,members.postal_code,members.phone01,members.tel_index,members.member_type,members.easn_member_id,members.keywords03,members.keywords04,members.keywords05,members.blurb_title,members.blurb_text,members.web_address,members.main_category,members.job_name02,members.company

from schema1.members  WHERE ftsearch @-@ to_tsquery('personal & care &
Barbers') and (edited_time is not null or member_type =
'paid_business')) as t1 WHERE this_rank > 0.3 ORDER BY state, this_rank
DESC;

I did a REINDEX and ANALYZE on the table, disabled the seqscan and ran
the query again with the same result.

My question how do I make PostgreSQL always uses bitmap scan instead of
seq scan?

Any advice is very much appreciated.

Best Regards,
Ahmed Ossama

You cannot ALWAYS do an indexed scan - sometimes the only option for the
parser is to do a sequential scan (thus you can say "avoid unless you have
to" but you can never truly disable sequential scanning).

Given limited knowledge of full-text searching I cannot explain why this
specific query is unable (or unwilling) to use an index.  Since you have not
provided any index options for the list to consider we may even assume you
have NO indexes and thus none to choose from...

I'd suggest looking at chapters 11 (Indexes) and 12 (Full-Text) of the
documentation while awaiting further responses from the list (after you
describe what indexes you have already defined on relevant tables - and
maybe provide the raw query as well).

David J


-----Original Message-----
From: pgsql-general-owner@postgresql.org
[mailto:pgsql-general-owner@postgresql.org] On Behalf Of Ahmed Ossama
Sent: Monday, February 14, 2011 12:40 PM
To: pgsql-general@postgresql.org
Subject: [GENERAL] Using Bitmap scan instead of Seq scan

I did a REINDEX and ANALYZE on the table, disabled the seqscan and ran the
query again with the same result.

My question how do I make PostgreSQL always uses bitmap scan instead of seq
scan?

Any advice is very much appreciated.

Best Regards,
Ahmed Ossama

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Jon Nelson <jnelson+pgsql@jamponi.net> writes:
> Does this suggest that the config items "disable_seqscan" (and friends)
> should be renamed to "avoid_seqscan" ?

Some of them actually are disable, some are only avoid.  The
documentation says which is which, but I don't think creating a
distinction at the naming level would be helpful (especially since
the planner's behavior has been known to change from time to time).

            regards, tom lane

On Mon, Feb 14, 2011 at 12:40 PM, Ahmed Ossama <ahmed@aossama.net> wrote:
> I did a REINDEX and ANALYZE on the table, disabled the seqscan and ran the
> query again with the same result.
>
> My question how do I make PostgreSQL always uses bitmap scan instead of seq
> scan?
>
> Any advice is very much appreciated.
>

Have a look at the config parameters related to sequential page cost
and random page cost. Perhaps you need to adjust the ratio between
them to better suit your hardware.  The planner may nudge itself over
to the index scan if it thinks it won't be too expensive.

Also, I'm not sure the default sample size for analyze in 8.4, but it
should probably be set to 100 if it is not already.