Write Skew

PostgreSQL 9.6 and 10 can use all three join strategies in parallel query plans, but they can only use a partial plan on the outer side of the join.  As of commit 18042840 , assuming nothing irreparably busted is discovered in the next few months, PostgreSQL 11 will ship with Parallel Hash.  Partial plans will be possible on both sides of a join for the first time. There will certainly be some adjustments  before it's released, but it seems like a good time to write a blog article to present Parallel Hash.  This is the biggest feature I've worked on in PostgreSQL so far, and I'm grateful to the reviewers, testers, committers and mentors of the PostgreSQL hacker community and EnterpriseDB for making this work possible. So what does this feature really do? A simple example Using the "orders" and "lineitem" tables from TPC-H scale 30GB, here is a very simple join query answering the (somewhat contrived) question "how many lineitems have there ev...