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** Find commmonly accessed tables and their use of indexes:

SELECT relname,seq_tup_read,idx_tup_fetch,cast(idx_tup_fetch AS numeric) / (idx_tup_fetch + seq_tup_read) AS idx_tup_pct FROM pg_stat_user_tables WHERE (idx_tup_fetch + seq_tup_read)>0 ORDER BY idx_tup_pct;

Returns output like:

       relname        | seq_tup_read | idx_tup_fetch |      idx_tup_pct       
----------------------+--------------+---------------+------------------------
 schema_migrations    |          817 |             0 | 0.00000000000000000000
 user_device_photos   |          349 |             0 | 0.00000000000000000000
 albums               |       530701 |           379 | 0.00071364012954733750
 facebook_oauths      |        15250 |            36 | 0.00235509616642679576
 
 
Analysis: For each row, because "idx_tup_pct" is low than it means that essentially no indexes are being used. In the case of "facebook_oauths"
 it turns out we are commonly running a query like "SELECT * FROM facebook_oauths WHERE fb_user_id = X" and it turns out there isnt an index on "fb_user_id"
 
 
 
===============================================================================

** Find the INSERT/UPDATE/DELETE statistics for tables:

SELECT relname,cast(n_tup_ins AS numeric) / (n_tup_ins + n_tup_upd + n_tup_del) AS ins_pct,cast(n_tup_upd AS numeric) / (n_tup_ins + n_tup_upd + n_tup_del) AS upd_pct, cast(n_tup_del AS numeric) / (n_tup_ins
+ n_tup_upd + n_tup_del) AS del_pct 
FROM pg_stat_user_tables 
WHERE (n_tup_ins + n_tup_upd + n_tup_del) > 0
ORDER BY relname;


================================================================================

** Table I/O

SELECT relname,cast(heap_blks_hit as numeric) / (heap_blks_hit + heap_blks_read) AS hit_pct,
heap_blks_hit,heap_blks_read 
FROM pg_statio_user_tables WHERE (heap_blks_hit + heap_blks_read)>0 ORDER BY hit_pct;

'heap_blks_hit' = the number of blocks that were satisfied from the page cache
'heap_blks_read' = the number of blocks that had to hit disk/IO layer for reads

When 'heap_blks_hit' is significantly greater than 'heap_blks_read' than it means we have a well-cached DB and most of the queries can be satisfied from the cache

================================================================================

** Table & Index sizes

SELECT
    t.tablename,
    indexname,
    c.reltuples::integer AS num_rows,
    pg_size_pretty(pg_relation_size(quote_ident(t.tablename)::text)) AS table_size,
    pg_size_pretty(pg_relation_size(quote_ident(indexrelname)::text)) AS index_size,
    CASE WHEN x.is_unique = 1  THEN 'Y'
       ELSE 'N'
    END AS UNIQUE,
    idx_scan AS number_of_scans,
    idx_tup_read AS tuples_read,
    idx_tup_fetch AS tuples_fetched
FROM pg_tables t
LEFT OUTER JOIN pg_class c ON t.tablename=c.relname
LEFT OUTER JOIN
       (SELECT indrelid,
           max(CAST(indisunique AS integer)) AS is_unique
       FROM pg_index
       GROUP BY indrelid) x
       ON c.oid = x.indrelid
LEFT OUTER JOIN
    ( SELECT c.relname AS ctablename, ipg.relname AS indexname, x.indnatts AS number_of_columns, idx_scan, idx_tup_read, idx_tup_fetch,indexrelname FROM pg_index x
           JOIN pg_class c ON c.oid = x.indrelid
           JOIN pg_class ipg ON ipg.oid = x.indexrelid
           JOIN pg_stat_all_indexes psai ON x.indexrelid = psai.indexrelid )
    AS foo
    ON t.tablename = foo.ctablename
WHERE t.schemaname='public'
ORDER BY pg_relation_size(quote_ident(indexrelname)::text) desc;


================================================================================

** Index Health

SELECT indexrelname,cast(idx_tup_read AS numeric) / idx_scan AS avg_tuples,idx_scan,idx_tup_read FROM pg_stat_user_indexes WHERE idx_scan > 0;

** Index Size

SELECT
  schemaname,
  relname,
  indexrelname,
  idx_scan,
 pg_size_pretty(pg_relation_size(i.indexrelid)) AS index_size
FROM
  pg_stat_user_indexes i
  JOIN pg_index USING (indexrelid)
WHERE
  indisunique IS false
ORDER BY idx_scan,relname;


** Index I/O - Same idea as Table I/O above

SELECT indexrelname,cast(idx_blks_hit as numeric) / (idx_blks_hit + idx_blks_read) AS hit_pct,
idx_blks_hit,idx_blks_read FROM pg_statio_user_indexes WHERE 
(idx_blks_hit + idx_blks_read)>0 ORDER BY hit_pct;


** Show sizes & usage of indexes that are not used very often:
NOTE: we define 'usage' by # of times used, in this case we use '200' - change accordingly

SELECT idstat.relname AS table_name, indexrelname AS index_name, idstat.idx_scan AS times_used,
pg_size_pretty(pg_relation_size(tabstat.relid)) AS table_size, pg_size_pretty(pg_relation_size(indexrelid)) AS index_size,
n_tup_upd + n_tup_ins + n_tup_del as num_writes, indexdef AS definition
FROM pg_stat_user_indexes AS idstat JOIN pg_indexes ON indexrelname = indexname
JOIN pg_stat_user_tables AS tabstat ON idstat.relname = tabstat.relname
WHERE idstat.idx_scan < 200 AND indexdef !~* 'unique'
ORDER BY idstat.relname, indexrelname;