Mastering PostgreSQL 12 - Third Edition

Parallelism in practice

Now that we've introduced the basics of parallelism, we have to learn what it means in the real world. Let's take a look at the following query:

test=# explain SELECT * FROM t_parallel;                           QUERY PLAN ---------------------------------------------------------------------- Seq Scan on t_parallel (cost=0.00..360621.20 rows=25000120 width=4)(1 row)

Why does PostgreSQL not use a parallel query? The table is sufficiently large and the worker PostgreSQL is available, so why doesn't it use a parallel query? The answer is that interprocess communication is really expensive. If PostgreSQL has to ship rows between processes, a query can actually be slower than in single-process mode. The optimizer uses cost parameters to punish interprocess ...