A health check playbook for your Postgres database

I talk with a lot of folks that set their database up, start working with it, and then are surprised by issues that suddenly crop up out of nowhere. The reality is, so many don’t want to have to be a DBA, instead you would rather build features and just have the database work. But your is that a database is a living breathing thing. As the data itself changes what is the right way to query and behave changes. Making sure your database is healthy and performing at it’s maximum level doesn’t require a giant overhaul constantly. In fact you can probably view it similar to how you approach personal health. Regular check-ups allow you to make small but important adjustments without having to make dramatic life altering changes to keep you on the right path.

After years of running and managing literally millions of Postgres databases, here’s my breakdown of what your regular Postgres health check should look like. Consider running this on a monthly basis to be able to make small tweaks and adjustments and avoid the drastic changes.

Cache rules everything around me

For many applications not all the data is accessed all the time. Instead certain datasets are accessed one and then for some period of time, then the data you’re accessing changes. Postgres in fact is quite good at keeping frequently accessed data in memory.

Your cache hit ratio tells you how often your data is served from in memory vs. having to go to disk. Serving from memory vs. going to disk will be orders of magnitude faster, thus the more you can keep in memory the better. Of course you could provision an instance with as much memory as you have data, but you don’t necessarily have to. Instead watching your cache hit ratio and ensuring it is at 99% is a good metric for proper performance.

You can monitor your cache hit ratio with:

SELECT 
  sum(heap_blks_read) as heap_read,
  sum(heap_blks_hit)  as heap_hit,
  sum(heap_blks_hit) / (sum(heap_blks_hit) + sum(heap_blks_read)) as ratio
FROM 
  pg_statio_user_tables;

Be careful of dead tuples

Under the covers Postgres is essentially a giant append only log. When you write data it appends to the log, when you update data it marks the old record as invalid and writes a new one, when you delete data it just marks it invalid. Later Postgres comes through and vacuums those dead records (also known as tuples).

All those unvacuumed dead tuples are what is known as bloat. Bloat can slow down other writes and create other issues. Paying attention to your bloat and when it is getting out of hand can be key for tuning vacuum on your database.

WITH constants AS (
  SELECT current_setting('block_size')::numeric AS bs, 23 AS hdr, 4 AS ma
), bloat_info AS (
  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, constants
    GROUP BY 1,2,3,4,5
  ) AS foo
), table_bloat AS (
  SELECT
    schemaname, tablename, 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
  FROM bloat_info
  JOIN pg_class cc ON cc.relname = bloat_info.tablename
  JOIN pg_namespace nn ON cc.relnamespace = nn.oid AND nn.nspname = bloat_info.schemaname AND nn.nspname <> 'information_schema'
), index_bloat AS (
  SELECT
    schemaname, tablename, bs,
    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 bloat_info
  JOIN pg_class cc ON cc.relname = bloat_info.tablename
  JOIN pg_namespace nn ON cc.relnamespace = nn.oid AND nn.nspname = bloat_info.schemaname AND nn.nspname <> 'information_schema'
  JOIN pg_index i ON indrelid = cc.oid
  JOIN pg_class c2 ON c2.oid = i.indexrelid
)
SELECT
  type, schemaname, object_name, bloat, pg_size_pretty(raw_waste) as waste
FROM
(SELECT
  'table' as type,
  schemaname,
  tablename as object_name,
  ROUND(CASE WHEN otta=0 THEN 0.0 ELSE table_bloat.relpages/otta::numeric END,1) AS bloat,
  CASE WHEN relpages < otta THEN '0' ELSE (bs*(table_bloat.relpages-otta)::bigint)::bigint END AS raw_waste
FROM
  table_bloat
    UNION
SELECT
  'index' as type,
  schemaname,
  tablename || '::' || iname as object_name,
  ROUND(CASE WHEN iotta=0 OR ipages=0 THEN 0.0 ELSE ipages/iotta::numeric END,1) AS bloat,
  CASE WHEN ipages < iotta THEN '0' ELSE (bs*(ipages-iotta))::bigint END AS raw_waste
FROM
  index_bloat) bloat_summary
ORDER BY raw_waste DESC, bloat DESC

Query courtesy of Heroku’s pg-extras

Over optimizing is a thing

We always our database to be performant, so in order to do that we keep things in memory/cache (see earlier) and we index things so we don’t have to scan everything on disk. But there is a trade-off when it comes to indexing your database. Each index the system has to maintain will slow down your write throughput on the database. This is fine when you do need to speed up queries, as long as they’re being utilized. If you added an index years ago, but something within your application changed and you no longer need it best to remove it.

Postgres makes it simply to query for unused indexes so you can easily give yourself back some performance by removing them:

SELECT
  schemaname || '.' || relname AS table,
  indexrelname AS index,
  pg_size_pretty(pg_relation_size(i.indexrelid)) AS index_size,
  idx_scan as index_scans
FROM pg_stat_user_indexes ui
JOIN pg_index i ON ui.indexrelid = i.indexrelid
WHERE NOT indisunique AND idx_scan < 50 AND pg_relation_size(relid) > 5 * 8192
ORDER BY pg_relation_size(i.indexrelid) / nullif(idx_scan, 0) DESC NULLS FIRST,
pg_relation_size(i.indexrelid) DESC;

Check in on your query performance

In an earlier blog post we talked about how useful pg_stat_statements was for monitoring your database query performance. It records a lot of valuable stats about which queries are run, how fast they return, how many times their run, etc. Checking in on this set of queries regularly can tell you where is best to add indexes or optimize your application so your query calls may not be so excessive.

Thanks to a HN commenter on our earlier post we have a great query that is easy to tweak to show different views based on all that data:

SELECT query, 
       calls, 
       total_time, 
       total_time / calls as time_per, 
       stddev_time, 
       rows, 
       rows / calls as rows_per,
       100.0 * shared_blks_hit / nullif(shared_blks_hit + shared_blks_read, 0) AS hit_percent
FROM pg_stat_statements
WHERE query not similar to '%pg_%'
and calls > 500
--ORDER BY calls
--ORDER BY total_time
order by time_per
--ORDER BY rows_per
DESC LIMIT 20;

An apple a day…

A regular health check for your database saves you a lot of time in the long run. It allows you to gradually maintain and improve things without huge re-writes. I’d personally recommend a monthly or bimonthly check-in on all of the above to ensure things are in a good state.