Solved (was: Re: Another index question)

Line noise, sorry...

After posting I went back to reading the pgsql docs and saw the query:

SELECT am.amname AS index_method, opc.opcname AS opclass_name,
opr.oprname AS opclass_operator FROM pg_am am, pg_opclass opc, pg_amop
amop, pg_operator opr WHERE opc.opcamid = am.oid AND amop.amopclaid =
opc.oid AND amop.amopopr = opr.oid ORDER BY index_method, opclass_name,
opclass_operator;

Which listed all the op_classes. I noticed none of the
opclass_operators supported '!=' so I wondered if that was simply an
unindexable (is that a word?) operator. So I tried creating the index:

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

And changing my query to:

tle-bu=> EXPLAIN ANALYZE UPDATE file_info_2 SET file_backup='i' WHERE
file_backup='t' OR file_backup='f' AND file_parent_dir~'^/';

QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------------
Index Scan using file_info_2_mupdate_idx, file_info_2_mupdate_idx on
file_info_2 (cost=0.00..10.04 rows=1 width=134) (actual
time=0.112..0.718 rows=4 loops=1)
Index Cond: ((file_backup = 't'::bpchar) OR ((file_backup =
'f'::bpchar) AND (file_parent_dir ~>=~ '/'::text) AND (file_parent_dir
~<~ '0'::text)))
Filter: ((file_backup = 't'::bpchar) OR ((file_backup = 'f'::bpchar)
AND (file_parent_dir ~ '^/'::text)))
Total runtime: 60.359 ms
(4 rows)

Bingo!

Hopefully someone might find this useful in the archives. :p

Madison

Madison Kelly wrote:
> 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