After learning about techniques for a single database we’ll use Citus to demonstrate how to parallelize counts in a distributed database. We’ll analyze the techniques available for each situation and compare their speed and resource consumption. Next, are you counting duplicates or just distinct values? Finally do you want a lump count of an entire table or will you want to count only those rows matching extra criteria? First think whether you need an exact count or whether an estimate suffices. The problem is actually underdescribed-there are several variations of counting, each with its own methods. If you know the tricks there are ways to count rows orders of magnitude faster than you do already. This article is a close look into how PostgreSQL optimizes counting. WHERE oid = 'public.Everybody counts, but not always quickly. Using pg_class: SELECT reltuples::bigint AS EstimatedCount You can use below to query to find row count. In fact, in my application, as we added joins and complex conditions, it became so inaccurate it was completely worthless, even to know how within a power of 100 how many rows we'd have returned, so we had to abandon that strategy.īut if your query is simple enough that Pg can predict within some reasonable margin of error how many rows it will return, it may work for you. You can use the rows=(\d+) value as a rough estimate of the number of rows that would be returned, then only do the actual SELECT COUNT(*) if the estimate is, say, less than 1.5x your threshold (or whatever number you deem makes sense for your application).ĭepending on the complexity of your query, this number may become less and less accurate. For a simple SELECT *, the first line of output should look something like this: Seq Scan on uids (cost=0.00.1.21 rows=8 width=75) Then examining the output with a regex, or similar logic. I did this once in a postgres app by running: EXPLAIN SELECT * FROM foo Not nearly as fast as the estimate in pg_class, though. Postgres actually stops counting beyond the given limit, you get an exact and current count for up to n rows (500000 in the example), and n otherwise. You can use a subquery with LIMIT: SELECT count(*) FROM (SELECT 1 FROM token LIMIT 500000) t Stop the counting (and not wait to finish the counting to inform the is possible at the moment the count pass my constant value, it will Answer to actual questionįirst, I need to know the number of rows in that table, if the totalĬount is greater than some predefined constant, In most cases the estimate from pg_class will be faster and more accurate. If unevenly distributed across the table, the estimate may be off.
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