Slow Counting

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Revision as of 17:37, 19 August 2010 by Nicolas.barbier (talk | contribs)
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One operation that PostgreSQL is known to be slow performing is doing a full count of rows in a table, typically using this SQL:


The reason why this is slow is related to the MVCC implementation in PostgreSQL. The fact that multiple transactions can see different states of the data means that there can be no straightforward way for "COUNT(*)" to summarize data across the whole table; PostgreSQL must walk through all rows, in some sense. This normally results in a sequential scan reading information about every row in the table. A good way to see what is going on with your query is to use EXPLAIN ANALYZE:

                                                      QUERY PLAN                                                       
 Aggregate  (cost=4499.00..4499.01 rows=1 width=0) (actual time=465.588..465.591 rows=1 loops=1)
   ->  Seq Scan on accounts  (cost=0.00..4249.00 rows=100000 width=0) (actual time=0.011..239.212 rows=100000 loops=1)
 Total runtime: 465.642 ms
(3 rows)

It is worth observing that it is only this precise form of aggregate that must be so pessimistic; if augmented with a "WHERE" clause like

SELECT COUNT(*) FROM table WHERE status = 'something'

PostgreSQL will take advantage of available indexes against the restricted field(s) to limit how many records must be counted, which can greatly accelerate such queries. PostgreSQL will still need to read the resulting rows to verify that they exist; other database systems may only need to reference the index in this situation.

Estimating the row count

One PostgreSQL alternative when only an approximate count is needed is to use the reltuples field from the pg_class catalog table:

pgbench=# select reltuples from pg_class where relname='tellers';

This presumes you have been running ANALYZE on the table enough to keep these statistics up to date.

Another popular approach is to use a trigger-based mechanism to count the rows in the table. One or both of these techniques are covered in the following: