Message-ID: <3F144838.8070209@joeconway.com>
Date: Tue, 15 Jul 2003 11:30:16 -0700
From: Joe Conway <mail@joeconway.com>
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To: David Olbersen <DOlbersen@stbernard.com>
Cc: pgsql-sql@postgresql.org
Subject: Re: Functional Indexes
References: <E7E213858379814A9AE48CA6754F5ECB0D6FC9@mail01.stbernard.com>
In-Reply-To: <E7E213858379814A9AE48CA6754F5ECB0D6FC9@mail01.stbernard.com>
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David Olbersen wrote:
> Now the question: is there a single index I can create that will be
> used when my  WHERE clause contains either urlhost or urltld? I could
> create two functional indexes, but that seems a bit silly to me.
> 

I can't think of how to do only one index in 7.3.x and earlier, but 
FWIW, this works in 7.4devel (which should be in beta next Monday):

create or replace function tld(text) returns text as '
select split_part(split_part(substr($1,8,length($1)),''/'',1),''.'',3)
' language 'sql' STRICT IMMUTABLE;

regression=# select tld('http://www.foobar.com/really/long/path/to/a/file');
  tld
-----
  com
(1 row)

create or replace function sld(text) returns text as '
select split_part(split_part(substr($1,8,length($1)),''/'',1),''.'',2)
' language 'sql' STRICT IMMUTABLE;

regression=# select sld('http://www.foobar.com/really/long/path/to/a/file');
   sld
--------
  foobar
(1 row)

create table urls(f1 text);
insert into urls values('http://www.foobar.com/really/long/path/to/a/file');
create index urls_idx1 on urls(tld(f1),sld(f1));

-- just to see index usage on toy table
set enable_seqscan to off;

regression=# explain analyze select * from urls where tld(f1) = 'com';
QUERY PLAN
----------------------------------------------------------------------
  Index Scan using urls_idx1 on urls  (cost=0.00..4.69 rows=1 width=32) 
(actual time=0.07..0.07 rows=1 loops=1)
    Index Cond: (split_part(split_part(substr(f1, 8, length(f1)), 
'/'::text, 1), '.'::text, 3) = 'com'::text)
  Total runtime: 0.18 msec
(3 rows)

regression=# explain analyze select * from urls where tld(f1) = 'com' 
and sld(f1) = 'foobar';
 
                               QUERY PLAN
---------------------------------------------------------------------
  Index Scan using urls_idx1 on urls  (cost=0.00..4.70 rows=1 width=32) 
(actual time=0.08..0.09 rows=1 loops=1)
    Index Cond: ((split_part(split_part(substr(f1, 8, length(f1)), 
'/'::text, 1), '.'::text, 3) = 'com'::text) AND 
(split_part(split_part(substr(f1, 8, length(f1)), '/'::text, 1), 
'.'::text, 2) = 'foobar'::text))
  Total runtime: 0.21 msec
(3 rows)


Joe