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PostgreSQL and bloat

The bucardo project has released its nagios plugins for PostgreSQL and we can extract from them this nice view in order to check for table and index bloat into our PostgreSQL databases:

        schemaname, tablename, reltuples::bigint, relpages::bigint, otta,
        ROUND(CASE WHEN otta=0 THEN 0.0 ELSE sml.relpages/otta::numeric END,1) AS tbloat,
        relpages::bigint - otta AS wastedpages,
        bs*(sml.relpages-otta)::bigint AS wastedbytes,
        pg_size_pretty((bs*(relpages-otta))::bigint) AS wastedsize,
        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 ipages::bigint - iotta END AS wastedipages,
        CASE WHEN ipages < iotta THEN 0 ELSE bs*(ipages-iotta) END AS wastedibytes,
        CASE WHEN ipages < iotta THEN pg_size_pretty(0) ELSE pg_size_pretty((bs*(ipages-iotta))::bigint) END AS wastedisize
      FROM (
          schemaname, tablename, cc.reltuples, cc.relpages, bs,
            (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 (
            (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 (
              schemaname, tablename, hdr, ma, bs,
              SUM((1-null_frac)*avg_width) AS datawidth,
              MAX(null_frac) AS maxfracsum,
                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 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
        LEFT JOIN pg_index i ON indrelid = cc.oid
        LEFT JOIN pg_class c2 ON c2.oid = i.indexrelid
      ) AS sml
      WHERE sml.relpages - otta > 0 OR ipages - iotta > 10
      ORDER BY wastedbytes DESC, wastedibytes DESC;

A relation is said to contain bloat as soon there are dead tuples no more referenced into it. Depending on your FSM settings you may or may not be able to clean out the bloat by running VACUUM on the bloated relations.

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