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

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 an aggregate function work in PostgreSQL?

Unlike most built-in aggregates, these aggregates are not strict, that is they do not drop input rows containing nulls. Null values sort according to the rule specified in the ORDER BY clause. Table 9-53. Grouping Operations GROUPING (args…)

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 kind of query does PostgreSQL do?

A query like: will require effort proportional to the size of the table: PostgreSQL will need to scan either the entire table or the entirety of an index which includes all rows in the table.

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 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.

How to avoid joins on a large table?

Tl;dr: Avoid joins on large tables and evaluate parts of queries beforehand to get 100–10,000x performance gains! As mentioned in a previous post, because of some of our tables growing in size, our queries started performing poorly which resulted in a performance hit to our most used APIs.

How big is a table in PostgreSQL in GB?

Earlier this week the performance of one of our (many) databases was plagued by a few pathologically large, primary-key queries in a smallish table (10 GB, 15 million rows) used to feed our graph editor. In 99.9% of accounts these queries would be zippy.

How to detect slow queries in PostgreSQL-cybertec?

Here is the idea: If a query exceeds a certain threshold, PostgreSQL can send the plan to the logfile for later inspection. The LOAD command will load the auto_explain module into a database connection. For the demo we can do that easily. In a production system one would use postgresql.conf or ALTER DATABASE / ALTER TABLE to load the module.

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.

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.