Re: Simple Query Elapsed Time ~ 3 hrs Using Bitmap Index/Heap Scan

From: Tom Lane
Subject: Re: Simple Query Elapsed Time ~ 3 hrs Using Bitmap Index/Heap Scan
Date: ,
Msg-id: 28448.1528209742@sss.pgh.pa.us
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In response to: Simple Query Elapsed Time ~ 3 hrs Using Bitmap Index/Heap Scan  (Fred Habash)
List: pgsql-performance


Fred Habash <> writes:
> Indexes:
>      "cl_pk"  PRIMARY KEY, btree (cl_id)
>      "cl_cnst_uk01"  UNIQUE CONSTRAINT, btree (cit_id, vclf_number, cl_value)
>      "cl_indx_fk01"  btree (cit_id)
>      "cl_indx_fk02"  btree (vclf_number)

This is pretty inefficient index design.  Your query is slow because the
only selective condition it has is on cl_value, but you have no index
that can be searched with cl_value as the leading condition.  Moreover,
you have two indexes that can be searched with cit_id as the leading
condition, which is just wasteful.  I'd try reorganizing the cl_cnst_uk01
index as (cl_value, vclf_number, cit_id) so that it can serve for
searches on cl_value, while still enforcing the same uniqueness condition.
This particular column ordering would also let your query use the
vclf_number constraint as a secondary search condition, which would
help even more.

There's relevant advice about index design in the manual,

https://www.postgresql.org/docs/current/static/indexes.html

(see 11.3 and 11.5 particularly)

            regards, tom lane



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From: Fd Habash
Date:
Subject: RE: Simple Query Elapsed Time ~ 3 hrs Using Bitmap Index/Heap Scan
From: e-blokos
Date:
Subject: Re: Client Server performance & UDS