Another index question - Mailing list pgsql-performance

Hi all,

   I am trying to do an update on a table but so far I can't seem to
come up with a usable index. After my last question/thread the user
'PFC' recommended I store whether a file was to be backed up as either
't'(rue), 'f'(alse) or 'i'(nherit) to speed up changing files and sub
directories under a given directory when it was toggled. I've more or
less finished implementing this and it is certainly a LOT faster but I
am hoping to make it just a little faster still with an Index.

   Tom Lane pointed out to me that I needed 'text_pattern_ops' on my
'file_parent_dir' column in the index if I wanted to do pattern matching
(the C locale wasn't set). Now I have added an additional condition and
I think this might be my problem. Here is a sample query I am trying to
create my index for:


UPDATE file_info_2 SET file_backup='i' WHERE file_backup!='i' AND
file_parent_dir='/';

   This would be an example of someone changing the backup state of the
root of a partition. It could also be:


UPDATE file_info_2 SET file_backup='i' WHERE file_backup!='i' AND
file_parent_dir='/usr';

   If, for example, the user was toggling the backup state of the '/usr'
directory.

   I suspected that because I was using "file_backup!='i'" that maybe I
was running into the same problem as before so I tried creating the index:


tle-bu=> CREATE INDEX file_info_2_mupdate_idx ON file_info_2
(file_backup bpchar_pattern_ops, file_parent_dir text_pattern_ops);

tle-bu=> EXPLAIN ANALYZE UPDATE file_info_2 SET file_backup='i' WHERE
file_backup!='i' AND file_parent_dir~'^/';
                       QUERY PLAN
-----------------------------------------------------------------------------------------------------------------
  Seq Scan on file_info_2  (cost=0.00..13379.38 rows=1 width=134)
(actual time=1623.819..1624.087 rows=4 loops=1)
    Filter: ((file_backup <> 'i'::bpchar) AND (file_parent_dir ~
'^/'::text))
  Total runtime: 1628.053 ms
(3 rows)


   This index wasn't used though, even when I set 'enable_seqscan' to
'OFF'. The column 'file_backup' is 'char(1)' and the column
'file_parent_dir' is 'text'.


tle-bu=> \d file_info_2; \di file_info_2_mupdate_idx;
Table "public.file_info_2"
      Column      |     Type     |          Modifiers
-----------------+--------------+------------------------------
  file_group_name | text         |
  file_group_uid  | integer      | not null
  file_mod_time   | bigint       | not null
  file_name       | text         | not null
  file_parent_dir | text         | not null
  file_perm       | integer      | not null
  file_size       | bigint       | not null
  file_type       | character(1) | not null
  file_user_name  | text         |
  file_user_uid   | integer      | not null
  file_backup     | character(1) | not null default 'i'::bpchar
  file_display    | character(1) | not null default 'i'::bpchar
  file_restore    | character(1) | not null default 'i'::bpchar
Indexes:
     "file_info_2_mupdate_idx" btree (file_backup bpchar_pattern_ops,
file_parent_dir text_pattern_ops)
     "file_info_2_supdate_idx" btree (file_parent_dir, file_name, file_type)

                         List of relations
  Schema |          Name           | Type  |  Owner  |    Table
--------+-------------------------+-------+---------+-------------
  public | file_info_2_mupdate_idx | index | madison | file_info_2
(1 row)

   Could it be that there needs to be a certain number of
"file_backup!='i'" before the planner will use the index? I have also
tried not defining an op_class on both tables (and one at a time) but I
can't seem to figure this out.

   As always, thank you!

Madison