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-- new table bloat query
-- still needs work; is often off by +/- 20%
WITH constants AS (
-- define some constants for sizes of things
-- for reference down the query and easy maintenance
SELECT current_setting('block_size')::numeric AS bs, 23 AS hdr, 8 AS ma
),
no_stats AS (
-- screen out table who have attributes
-- which dont have stats, such as JSON
SELECT table_schema, table_name,
n_live_tup::numeric as est_rows,
pg_table_size(relid)::numeric as table_size
FROM information_schema.columns
JOIN pg_stat_user_tables as psut
ON table_schema = psut.schemaname
AND table_name = psut.relname
LEFT OUTER JOIN pg_stats
ON table_schema = pg_stats.schemaname
AND table_name = pg_stats.tablename
AND column_name = attname
WHERE attname IS NULL
AND table_schema NOT IN ('pg_catalog', 'information_schema')
GROUP BY table_schema, table_name, relid, n_live_tup
),
null_headers AS (
-- calculate null header sizes
-- omitting tables which dont have complete stats
-- and attributes which aren't visible
SELECT
hdr+1+(sum(case when null_frac <> 0 THEN 1 else 0 END)/8) as nullhdr,
SUM((1-null_frac)*avg_width) as datawidth,
MAX(null_frac) as maxfracsum,
schemaname,
tablename,
hdr, ma, bs
FROM pg_stats CROSS JOIN constants
LEFT OUTER JOIN no_stats
ON schemaname = no_stats.table_schema
AND tablename = no_stats.table_name
WHERE schemaname NOT IN ('pg_catalog', 'information_schema')
AND no_stats.table_name IS NULL
AND EXISTS ( SELECT 1
FROM information_schema.columns
WHERE schemaname = columns.table_schema
AND tablename = columns.table_name )
GROUP BY schemaname, tablename, hdr, ma, bs
),
data_headers AS (
-- estimate header and row size
SELECT
ma, bs, hdr, 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 null_headers
),
table_estimates AS (
-- make estimates of how large the table should be
-- based on row and page size
SELECT schemaname, tablename, bs,
reltuples::numeric as est_rows, relpages * bs as table_bytes,
CEIL((reltuples*
(datahdr + nullhdr2 + 4 + ma -
(CASE WHEN datahdr%ma=0
THEN ma ELSE datahdr%ma END)
)/(bs-20))) * bs AS expected_bytes,
reltoastrelid
FROM data_headers
JOIN pg_class ON tablename = relname
JOIN pg_namespace ON relnamespace = pg_namespace.oid
AND schemaname = nspname
WHERE pg_class.relkind = 'r'
),
estimates_with_toast AS (
-- add in estimated TOAST table sizes
-- estimate based on 4 toast tuples per page because we dont have
-- anything better. also append the no_data tables
SELECT schemaname, tablename,
TRUE as can_estimate,
est_rows,
table_bytes + ( coalesce(toast.relpages, 0) * bs ) as table_bytes,
expected_bytes + ( ceil( coalesce(toast.reltuples, 0) / 4 ) * bs ) as expected_bytes
FROM table_estimates LEFT OUTER JOIN pg_class as toast
ON table_estimates.reltoastrelid = toast.oid
AND toast.relkind = 't'
),
table_estimates_plus AS (
-- add some extra metadata to the table data
-- and calculations to be reused
-- including whether we cant estimate it
-- or whether we think it might be compressed
SELECT current_database() as databasename,
schemaname, tablename, can_estimate,
est_rows,
CASE WHEN table_bytes > 0
THEN table_bytes::NUMERIC
ELSE NULL::NUMERIC END
AS table_bytes,
CASE WHEN expected_bytes > 0
THEN expected_bytes::NUMERIC
ELSE NULL::NUMERIC END
AS expected_bytes,
CASE WHEN expected_bytes > 0 AND table_bytes > 0
AND expected_bytes <= table_bytes
THEN (table_bytes - expected_bytes)::NUMERIC
ELSE 0::NUMERIC END AS bloat_bytes
FROM estimates_with_toast
UNION ALL
SELECT current_database() as databasename,
table_schema, table_name, FALSE,
est_rows, table_size,
NULL::NUMERIC, NULL::NUMERIC
FROM no_stats
),
bloat_data AS (
-- do final math calculations and formatting
select current_database() as databasename,
schemaname, tablename, can_estimate,
table_bytes, round(table_bytes/(1024^2)::NUMERIC,3) as table_mb,
expected_bytes, round(expected_bytes/(1024^2)::NUMERIC,3) as expected_mb,
round(bloat_bytes*100/table_bytes) as pct_bloat,
round(bloat_bytes/(1024::NUMERIC^2),2) as mb_bloat,
table_bytes, expected_bytes, est_rows
FROM table_estimates_plus
)
-- filter output for bloated tables
SELECT databasename, schemaname, tablename,
can_estimate,
est_rows,
pct_bloat, mb_bloat,
table_mb
FROM bloat_data
-- this where clause defines which tables actually appear
-- in the bloat chart
-- example below filters for tables which are either 50%
-- bloated and more than 20mb in size, or more than 25%
-- bloated and more than 4GB in size
WHERE ( pct_bloat >= 50 AND mb_bloat >= 10 )
OR ( pct_bloat >= 25 AND mb_bloat >= 1000 )
ORDER BY pct_bloat DESC;
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