Re: Index Bloat - how to tell?

I have used this in the past ... run this against the database that you want to inspect.


SELECT
 current_database(), schemaname, tablename, /*reltuples::bigint, relpages::bigint, otta,*/
 ROUND(CASE WHEN otta=0 THEN 0.0 ELSE sml.relpages/otta::numeric END,1) AS tbloat,
 CASE WHEN relpages < otta THEN 0 ELSE bs*(sml.relpages-otta)::bigint END AS wastedbytes,
 iname, /*ituples::bigint, ipages::bigint, iotta,*/
 ROUND(CASE WHEN iotta=0 OR ipages=0 THEN 0.0 ELSE ipages/iotta::numeric END,1) AS ibloat,
 CASE WHEN ipages < iotta THEN 0 ELSE bs*(ipages-iotta) END AS wastedibytes
FROM (
 SELECT
   schemaname, tablename, cc.reltuples, cc.relpages, bs,
   CEIL((cc.reltuples*((datahdr+ma-
     (CASE WHEN datahdr%ma=0 THEN ma ELSE datahdr%ma END))+nullhdr2+4))/(bs-20::float)) AS otta,
   COALESCE(c2.relname,'?') AS iname, COALESCE(c2.reltuples,0) AS ituples, COALESCE(c2.relpages,0) AS ipages,
   COALESCE(CEIL((c2.reltuples*(datahdr-12))/(bs-20::float)),0) AS iotta -- very rough approximation, assumes all cols
 FROM (
   SELECT
     ma,bs,schemaname,tablename,
     (datawidth+(hdr+ma-(case when hdr%ma=0 THEN ma ELSE hdr%ma END)))::numeric AS datahdr,
     (maxfracsum*(nullhdr+ma-(case when nullhdr%ma=0 THEN ma ELSE nullhdr%ma END))) AS nullhdr2
   FROM (
     SELECT
       schemaname, tablename, hdr, ma, bs,
       SUM((1-null_frac)*avg_width) AS datawidth,
       MAX(null_frac) AS maxfracsum,
       hdr+(
         SELECT 1+count(*)/8
         FROM pg_stats s2
         WHERE null_frac<>0 AND s2.schemaname = s.schemaname AND s2.tablename = s.tablename
       ) AS nullhdr
     FROM pg_stats s, (
       SELECT
         (SELECT current_setting('block_size')::numeric) AS bs,
         CASE WHEN substring(v,12,3) IN ('8.0','8.1','8.2') THEN 27 ELSE 23 END AS hdr,
         CASE WHEN v ~ 'mingw32' THEN 8 ELSE 4 END AS ma
       FROM (SELECT version() AS v) AS foo
     ) AS constants
     GROUP BY 1,2,3,4,5
   ) AS foo
 ) AS rs
 JOIN pg_class cc ON cc.relname = rs.tablename
 JOIN pg_namespace nn ON cc.relnamespace = nn.oid AND nn.nspname = rs.schemaname AND nn.nspname <> 'information_schema'
 LEFT JOIN pg_index i ON indrelid = cc.oid
 LEFT JOIN pg_class c2 ON c2.oid = i.indexrelid
) AS sml
ORDER BY wastedbytes DESC

-----Original Message-----
From: pgsql-performance-owner@postgresql.org [mailto:pgsql-performance-owner@postgresql.org] On Behalf Of John W Strange
Sent: Tuesday, December 14, 2010 8:48 AM
To: pgsql-performance@postgresql.org
Subject: [PERFORM] Index Bloat - how to tell?

How can you tell when your indexes are starting to get bloated and when you need to rebuild them.  I haven't seen a quick way to tell and not sure if it's being tracked.



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