Ignored btree indexes on particular tables.
Hi all
I've recently installed pg 8.2.5 on a new server and transferred my data
from 8.2.4 running on a slow old thing, via pg_dump.
One of these tables has point UK address data, with 27 million rows, and
another the UK roads data, approx 4 million rows.
My problem is I have several text fields in the address data, for which
postgres ignores the indexes (btree).
Using my pc_ (postcode) column:
Here's the OLD query plan on the old server.
"Index Scan using ap_idx_pc on ap (cost= 0.00..15.30 rows=1 width=188)"
" Index Cond: (((pc_)::text >= 'OX2 0'::character varying) AND ((pc_)::text
< 'OX2 1'::character varying))"
" Filter: ((pc_)::text ~~ 'OX2 0%'::text)"
And the NEW:-
"Seq Scan on ap (cost=0.00..4652339.33 rows=1 width=189)"
" Filter: ((pc_)::text ~~ 'OX2 0%'::text)"
I have tried :
reindexing.
dropping the index and recreating it.
set enable_seqscan = off;
set seq_page_cost = 1000;
vacuum analyze;
vacuum full;
none of these things have worked.
the strange thing is my btree indexes on the uk roads data work fine.
There are quite a few nulls in the table, but very few in the pc column I've
been using as an example.
Any help would be greatly appreciated.
Cheers
Will
William Temperley wrote:
My problem is I have several text fields in the address data, for which
postgres ignores the indexes (btree).
"Index Scan using ap_idx_pc on ap (cost= 0.00..15.30 rows=1 width=188)"
" Index Cond: (((pc_)::text >= 'OX2 0'::character varying) AND ((pc_)::text
< 'OX2 1'::character varying))"
" Filter: ((pc_)::text ~~ 'OX2 0%'::text)"And the NEW:-
"Seq Scan on ap (cost=0.00..4652339.33 rows=1 width=189)"
" Filter: ((pc_)::text ~~ 'OX2 0%'::text)"
It's almost certainly a locale thing. Your old locale was "C" and the
new one is "en_GB.UTF-8" or similar. This means that simple sorting has
been replaced by something more library-like.
You can either dump the database, re-run initdb with the "C" locale and
restore, or read up on text_pattern_ops/varchar_pattern_ops in the
manual (11.8. Operator Classes). Basically it tags an index as working
with pattern-matching in the current locale.
the strange thing is my btree indexes on the uk roads data work fine.
Do they use like, or explicit range-checks?
--
Richard Huxton
Archonet Ltd
Genius!
Thanks Richard,
The old locale was C and the new one English_United Kingdom.1252
I created a new index with "varchar_pattern_ops" and off it went!
the strange thing is my btree indexes on the uk roads data work fine.
Do they use like, or explicit range-checks?
I wasn't using like, which is why they were working. adding a new index
then using the like operator didn't work.
Thanks again.
Will