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Re: indexing for distinct search in timestamp based table

From: "Vladimir Sitnikov" <sitnikov(dot)vladimir(at)gmail(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: indexing for distinct search in timestamp based table
Date: 2008-09-05 15:10:35
Message-ID: 1d709ecc0809050810m42bfbedfk98ac7d83f13fb7@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
You might get great improvement for '%' cases using index on
channel_name(<field>,
start_time) and a little bit of pl/pgsql

Basically, you need to implement the following algorithm:
 1) curr_<field> = ( select  min(<field>) from ad_log )
 2) record_exists = ( select 1 from ad_log where <field>=cur_<field> and
_all_other_conditions limit 1 )
 3) if record_exists==1 then add curr_<field> to the results
 3) curr_<field> = (select min(<field>) from ad_log where <field>  >
 curr_<field> )
 4) if curr_<field> is not null then goto 2


I believe it might make sense implement this approach in the core (I would
call it "index distinct scan")

That could dramatically improve "select distinct <column> from <table>" and
"select <column> from <table> group by <column>" kind of queries when there
exists an index on <column> and a particular column has very small number of
distinct values.

For instance:  say a table has 10'000'000 rows, while column of interest has
only 20 distinct values. In that case, the database will be able to get
every of those 20 values in virtually 20 index lookups.

What does the community think about that?

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