PostgreSQL Common Utility Queries

basic.sql

/* How to calculate postgreSQL database size in disk ? */

SELECT pg_size_pretty(pg_database_size('thedbname'));

/* Calculate size of a table including or excluding the index */

SELECT pg_size_pretty(pg_total_relation_size('big_table'));
SELECT pg_size_pretty(pg_relation_size('big_table')); /* without index */

/* See indexes on a table with `\d tablename` */

biggest_relations.sql

/* Finding the total size of your biggest tables */

SELECT nspname || '.' || relname AS "relation",
    pg_size_pretty(pg_relation_size(C.oid)) AS "size"
  FROM pg_class C
  LEFT JOIN pg_namespace N ON (N.oid = C.relnamespace)
  WHERE nspname NOT IN ('pg_catalog', 'information_schema')
  ORDER BY pg_relation_size(C.oid) DESC
  LIMIT 20;

biggest_tables.sql

/* Finding the total size of your biggest tables */

SELECT nspname || '.' || relname AS "relation",
    pg_size_pretty(pg_total_relation_size(C.oid)) AS "total_size"
  FROM pg_class C
  LEFT JOIN pg_namespace N ON (N.oid = C.relnamespace)
  WHERE nspname NOT IN ('pg_catalog', 'information_schema')
    AND C.relkind <> 'i'
    AND nspname !~ '^pg_toast'
  ORDER BY pg_total_relation_size(C.oid) DESC
  LIMIT 20;

index_size.sql

/* Table & index sizes along which indexes are being scanned and how many tuples are fetched */

SELECT
    t.tablename,
    indexname,
    c.reltuples AS num_rows,
    pg_size_pretty(pg_relation_size(t.tablename::text)) AS table_size,
    pg_size_pretty(pg_relation_size(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 1,2;

index_summary.sql

/* Pull the number of rows, indexes, and some info about those indexes for each table */

SELECT
    pg_class.relname,
    pg_size_pretty(pg_class.reltuples::bigint) AS rows_in_bytes,
    pg_class.reltuples AS num_rows,
    count(indexname) AS number_of_indexes,
    CASE WHEN x.is_unique = 1 THEN 'Y'
       ELSE 'N'
    END AS UNIQUE,
    SUM(case WHEN number_of_columns = 1 THEN 1
              ELSE 0
            END) AS single_column,
    SUM(case WHEN number_of_columns IS NULL THEN 0
             WHEN number_of_columns = 1 THEN 0
             ELSE 1
           END) AS multi_column
FROM pg_namespace 
LEFT OUTER JOIN pg_class ON pg_namespace.oid = pg_class.relnamespace
LEFT OUTER JOIN
       (SELECT indrelid,
           max(CAST(indisunique AS integer)) AS is_unique
       FROM pg_index
       GROUP BY indrelid) x
       ON pg_class.oid = x.indrelid
LEFT OUTER JOIN
    ( SELECT c.relname AS ctablename, ipg.relname AS indexname, x.indnatts AS number_of_columns FROM pg_index x
           JOIN pg_class c ON c.oid = x.indrelid
           JOIN pg_class ipg ON ipg.oid = x.indexrelid  )
    AS foo
    ON pg_class.relname = foo.ctablename
WHERE 
     pg_namespace.nspname='public'
AND  pg_class.relkind = 'r'
GROUP BY pg_class.relname, pg_class.reltuples, x.is_unique
ORDER BY 2;

index_useless.sql

/* Finds indices that have not been used (the ones with 0 scans) */

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;

And debug information:

• Check your main GUC settings to make sure that they are set to sensible values (see Tuning Your PostgreSQL Server for additional hints):
  • shared_buffers should be 10% to 25% of available RAM
  • effective_cache_size should be 75% of available RAM
• Test changing work_mem: increase it to 8MB, 32MB, 256MB, 1GB. Does it make a difference?
• For Insert/Update/Delete queries, you should also try configuring your WAL:
  • Move pg_xlog to a separate disk resource, if possible
  • Increase checkpoint_segments to 16 or more (assuming you have disk space)
• Increase wal_buffers to 16MB

Also a useful article: http://craigkerstiens.com/2012/10/01/understanding-postgres-performance/