What is difference between merge join and hash join?

What is difference between merge join and hash join?

1. This means that in case no other join is preferred (maybe due to no sorting or indexing etc), then, Hash join is used. Hash join is best algorithm when large, unsorted, and non-indexed data (residing in tables) is to be joined.

How to make join faster in PostgreSQL?

An index on the sort keys can speed up sorting, so an index on the join keys on both relations can speed up a merge join. However, an explicit sort is often cheaper unless an index only scan can be used.

What is merge join in PostgreSQL?

Merge Join is an algorithm wherein each record of outer relation is matched with each record of inner relation until there is a possibility of join clause matching. This join algorithm is only used if both relations are sorted and join clause operator is “=”.

Why JOINs are costly?

Joins involving properly selected keys with correctly set up indexes are cheap, not expensive, because they allow significant pruning of the result before the rows are materialised. Materialising the result involves bulk disk reads which are the most expensive aspect of the exercise by an order of magnitude.

How does a single PostgreSQL config change improved slow query performance?

To see the alternative query plans PostgreSQL considered before picking Hash Join, I disabled hash join and reran the query. There you go! The same query finished 50x faster when using a Nested Loop instead of a Hash Join. So why did PostgreSQL choose a worse plan for app A?

How does nested loop improve PostgreSQL query performance?

Interestingly, app A only accessed 10x more data than app B, but the response time was 3000x longer. To see the alternative query plans PostgreSQL considered before picking Hash Join, I disabled hash join and reran the query. There you go! The same query finished 50x faster when using a Nested Loop instead of a Hash Join.

How does a sequential scan improve PostgreSQL query performance?

The PostgreSQL execution plan for this query was unexpected. Even though both tables have Indexes, PostgreSQL decided to do a Hash Join with a sequential scan on the large table. The sequential scan on a large table contributed to most of the query time.

What are the default values for PostgreSQL random page cost?

The default PostgreSQL values of 4 and 1 for ‘ random_page_cost ’, ‘ seq_page_cost ’ respectively are tuned for HDD, where random access to disk is more expensive than sequential access. However these costs were inaccurate for our deployment using gp2 EBS volume, which are solid state drives.