get-a-quick-approximate-count-of-a-table.md

Get A Quick Approximate Count Of A Table

Really large PostgreSQL tables can be slow to work with. Even a count of the rows in a really large table can take a while to tabulate. I'm talking about tables on the order of hundreds of millions of rows.

For instance, here is a query grabbing the count of a ~400 million row table.

> select count(*) from events;

   count
-----------
 427462316
(1 row)

Time: 55113.794 ms

If I'm willing to wait nearly a minute (55 seconds), I can get an accurate count of the rows in this events table.

If I don't want to wait and an approximate count will do, there are faster ways. One way is to query the pg_class table.

> select reltuples::numeric as count
  from pg_class
  where relname='events';

   count
-----------
 427462000
(1 row)

Time: 0.413 ms

The resulting count is within hundreds of the actual value and tells me what I need to know. And instead of 55 seconds, it takes less than half a millisecond.

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