Databases · Middle · Question 14 of 75

Why can `SELECT COUNT(*) FROM big_table` be slow in PostgreSQL?

1COUNT(*) is not supported on large tables
✓Correct answer: Because of MVCC each transaction may see different rows, so PostgreSQL has to scan to count visible ones
3PostgreSQL counts every column of every row

Explanation

If an approximate number is enough, read the estimate from pg_class.reltuples, or keep a counter updated by the application or a trigger.

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