Re: [GENERAL] bloat indexes - opinion - Mailing list pgsql-general

On 02/21/2017 03:41 PM, Patrick B wrote:
> 2017-02-22 11:11 GMT+13:00 Patrick B <patrickbakerbr@gmail.com
> <mailto:patrickbakerbr@gmail.com>>:
>
>     2017-02-22 10:59 GMT+13:00 Adrian Klaver <adrian.klaver@aklaver.com
>     <mailto:adrian.klaver@aklaver.com>>:

>
>         On 02/21/2017 01:44 PM, Patrick B wrote:
>         > Hi guys,
>         >
>         > I've got a lot of bloat indexes on my 4TB database.
>         >
>         > Let's take this example:
>         >
>         >     Table: seg
>         >     Index: ix_filter_by_tree
>         >     Times_used: 1018082183
>         >     Table_size: 18 GB -- wrong. The table is mostly on pg_toast table.
>         >     Its real size is 2TB
>
>         How do you know one number is right and the other is wrong?
>
>
>
>     1. on that table (seg) i store binary data. It is impossible to have
>     only 18GB of it.
>     2.
>
>             SELECT schema_name,
>
>                    pg_size_pretty(sum(table_size)::bigint),
>
>                    (sum(table_size) /
>             pg_database_size(current_database())) * 100
>
>             FROM (
>
>               SELECT pg_catalog.pg_namespace.nspname as schema_name,
>
>                      pg_relation_size(pg_catalog.pg_class.oid) as table_size
>
>               FROM   pg_catalog.pg_class
>
>                  JOIN pg_catalog.pg_namespace ON relnamespace =
>             pg_catalog.pg_namespace.oid
>
>             ) t
>
>             GROUP BY schema_name
>
>             ORDER BY schema_name
>
>

>     pg_toast2706 GB82.62112838877240860000 <-- this belongs to the seg
>     table.
>
>
>
>
>         Have you looked at the functions here?:
>         https://www.postgresql.org/docs/9.6/static/functions-admin.html#FUNCTIONS-ADMIN-DBOBJECT
>         <https://www.postgresql.org/docs/9.6/static/functions-admin.html#FUNCTIONS-ADMIN-DBOBJECT>
>
>         >     Index_size: 17 GB
>         >     Num_writes 16245023
>         >     Index definition: CREATE INDEX ix_filter_by_tree ON seg USING btree
>         >     (full_path varchar_pattern_ops) WHERE (full_path IS NOT NULL)
>         >
>         >
>         >
>         > What is the real impact of a bloat index? If I reindex it, queries will
>         > be faster?
>         >
>         > Thanks
>         > Patrick
>
>
>
>
> I ran the query before and after the reindex, and it seems it did not
> help on performance.
>
> *The query I used:*
>
>     explain analyze select * from seg where full_path = '/userfile/123';

The table schema would be useful.

>
>
> *Before reindex:*
>
>     Index Scan using ix_filter_by_tree on seg  (cost=0.00..144.87
>     rows=215 width=8) (actual time=0.047..0.047 rows=1 loops=1)
>       Index Cond: (full_path = '/userfile/123')
>     Total runtime: 0.059 ms
>     (3 rows)
>
>
> *After reindex:*
>
>     Index Scan using ix_filter_by_tree on seg  (cost=0.00..141.83
>     rows=220 width=8) (actual time=0.021..0.021 rows=1 loops=1)
>       Index Cond: (full_path = '/userfile/123')
>     Total runtime: 0.036 ms
>     (3 rows)

Not showing the complete explain analyze makes the above not all that enlightening.

>
>
> Note that the '*/cost/*' is pretty much the same.
>
> *My question is:*
> If I have a bloat index. Why do I need to reindex it if I got none
> performance improvements?

Because it is an indication that you may not have index bloat?

Not sure a runtime of 0.036 to 0.036 ms over a 2TB table is symptomatic of a problem.

Might be worth taking a look at:

https://www.postgresql.org/docs/9.6/static/monitoring-stats.html#PG-STAT-ALL-TABLES-VIEW