index bloat query understand - Mailing list pgsql-general

I got a query to calculate index bloat as follows:

http://pgsql.tapoueh.org/site/html/news/20080131.bloat.html

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CREATE OR REPLACE VIEW bloat AS
      SELECT
        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 (
        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
        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;


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Could anyone please describe me what are the meaning of the followings in the query:


# ma -  CASE WHEN v ~ 'mingw32' THEN 8 ELSE 4 END AS ma
# CASE WHEN substring(v,12,3) IN ('8.0','8.1','8.2') THEN 27 ELSE 23 END AS hdr
# COALESCE(CEIL((c2.reltuples*(datahdr-12))/(bs-20::float)),0) AS iotta
# (CASE WHEN datahdr%ma=0 THEN ma ELSE datahdr%ma END))+nullhdr2+4))/(bs-20::float)) AS otta