PostgreSQL的索引膨胀

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磨砺技术珠矶，践行数据之道，追求卓越价值

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索引膨胀，主要是针对B-tree而言。

索引膨胀的几个来源：

1 大量删除发生后，导致索引页面稀疏，降低了索引使用效率。

2 PostgresQL 9.0之前的版本，vacuum full 会同样导致索引页面稀疏。

3 长时间运行的事务，禁止vacuum对表的清理工作，因而导致页面稀疏状态一直保持。

如何找出膨胀的索引，参见：

https://wiki.postgresql.org/wiki/Index_Maintenance#Index_Bloat

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;