Re: Queries for unused/useless indexes

[William Dunn <dunnwjr@xxxxxxxxx>]

Melvin - thanks for sharing.

Here is the query I use which lists the percent of queries against the table which use the index ordered by least used first.

The 'pg_relation_size(relid)>(5*8192)' is used to remove any tables that would be so small the optimizer would just choose a table scan.

SELECT schemaname,

       relname,

       idx_scan,

       seq_scan,

       (100 * idx_scan / (seq_scan + idx_scan)) AS perc_idx_used

FROM pg_stat_user_tables

WHERE pg_relation_size(relid)>(5*8192)

  AND NOT ((idx_scan=0

            OR idx_scan=NULL)

           AND seq_scan=0)

ORDER BY perc_idx_used;

Will J. Dunn

willjdunn.com

On Mon, May 25, 2015 at 12:39 PM, Peter J. Holzer <hjp@xxxxxx> wrote:

On 2015-05-25 12:25:01 -0400, Melvin Davidson wrote:
> I'm not sure why you are using "pg_stat_user_indexes".

Because you did. I didn't change that.

> My original query below
> uses "pg_stat_all_indexes" and the schema names are joined and it does work.

I'm not sure what you mean by "original", but this:

> SELECT n.nspname as schema,
>        i.relname as table,
>        i.indexrelname as index,
>        i.idx_scan,
>        i.idx_tup_read,
>        i.idx_tup_fetch,
>        pg_size_pretty(pg_relation_size(quote_ident(n.nspname) || '.' ||
> quote_ident(i.relname))) AS table_size,
>        pg_size_pretty(pg_relation_size(quote_ident(n.nspname) || '.' ||
> quote_ident(i.indexrelname))) AS index_size,
>        pg_get_indexdef(idx.indexrelid) as idx_definition
>   FROM pg_stat_all_indexes i
>   JOIN pg_class c ON (c.oid = i.relid)
>   JOIN pg_namespace n ON (n.oid = c.relnamespace)
>   JOIN pg_index idx ON (idx.indexrelid =  i.indexrelid )
>  WHERE i.idx_scan < 200
>    AND NOT idx.indisprimary
>    AND NOT idx.indisunique
>  ORDER BY 1, 2, 3;

is not the query you posted in your original message.

Here is what you posted:

> On Mon, May 25, 2015 at 10:41 AM, Peter J. Holzer <hjp-pgsql@xxxxxx> wrote:
>
>     On 2015-05-22 09:41:57 -0400, Melvin Davidson wrote:
>     > I'd like to share those queries with the community, as I know there must
>     be
>     > others out there with the same problem.
>     >
>     > /* useless_indexes.sql */
>     > SELECT
>     >        idstat.schemaname AS schema,
>     >        idstat.relname AS table_name,
>     >        indexrelname AS index_name,
>     >        idstat.idx_scan AS times_used,
>     >        pg_size_pretty(pg_relation_size(quote_ident(idstat.schemaname) ||
>     '.' ||
>     > quote_ident(idstat.relname))) AS table_size,
>     >        pg_size_pretty(pg_relation_size(quote_ident(idstat.schemaname) ||
>     '.' ||
>     > quote_ident(indexrelname))) AS index_size,
>     >        n_tup_upd + n_tup_ins + n_tup_del as num_writes,
>     >        indexdef AS definition
>     > FROM pg_stat_user_indexes AS idstat
>     > JOIN pg_indexes ON indexrelname = indexname
>     > JOIN pg_stat_user_tables AS tabstat ON idstat.relname = tabstat.relname
>     > WHERE idstat.idx_scan < 200
>     > AND indexdef !~* 'unique'
>     > ORDER BY idstat.schemaname,
>     >          idstat.relname,
>     >          indexrelname;

--
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