Why is query time so slow in PostgreSQL?

Why is query time so slow in PostgreSQL?

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. I initially suspected it could be due to fragmentation.

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.

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.

What’s the difference between first and second PostgreSQL queries?

The queries are basically the same, but PostgreSQL will use totally different execution plans. The first query will only fetch a handful of rows and therefore go for an index scan. The second query will fetch all the data and therefore prefer a sequential scan.

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.

Is there a way to improve Postgres database performance?

It still has a very noticeable impact on Postgres performance. Remember that the query is quite simple. It’s a primary key lookup so there aren’t many obvious ways to fix it without dramatically re-architecting the database or the application. We turned to the community for help via the PGSQL-Performance mailing list.

Why does Postgres keep changing the number of keys?

Here is the surprisingly simple culprit: Table X contains a few thousand rows, Table C contains 15 millions rows. Both “key” columns are primary keys with proper indexing. Pretty straightforward, it’s a simple primary key lookup. Where things get interesting is when you increase the number of keys.

Is it too heavy to change PostgreSQL query log settings?

Note: It’s often too heavy for PostgreSQL infrastructure if you change slow query log settings in postgresql.conf , Therefore it makes more sensible to change only for a selected database or user:

What does it mean when a query is slow?

When working with OLTP (OnLine Transaction Processing) databases, query performance is paramount as it directly impacts the user experience. Slow queries mean that the application feels unresponsive and slow and this results in bad conversion rates, unhappy users, and all sets of problems.

When to dig into PostgreSQL server performance problems?

When digging into PostgreSQL performance it is always good to know, which option one has to spot performance problems and to figure out, what is really going on on a server. Finding slow queries and performance weak spots is therefore exactly what this post is all about.

What does it mean when a query is running in PostgreSQL?

If a query (or set of queries) in question has the status of ‘active’, then it’s actually running. If the whole query isn’t available in pg_stat_activity, fetch it from the developers or the postgresql log and start exploring the query planner.