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~'^/';

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)


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


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
"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!


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