Why are there no slow queries in PostgreSQL?

Why are there no slow queries in PostgreSQL?

All those queries will never show up in the slow query log because they are still considered to be “fast”. What you might find, however, consists of backups, CREATE INDEX, bulk loads and so on. You might never find the root cause if you only rely on the slow query log.

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’s the advantage of a slow query log?

The advantage of the slow query log is that you can instantly inspect a slow query. Whenever something is slow, you can respond instantly to any individual query, which exceeds the desired threshold. However, the strength of this approach is also its main weakness.

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.

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 to improve the performance of PostgreSQL query?

A query can be fast, but if you call it too many times, the total time will be high. In that case, you should investigate if bulking the calls is feasible. See more details in the following article: PostgreSQL Log Analysis with pgBadger. First you should pay attention to the quantity of rows and loops.

How many queries should be logged in PostgreSQL?

Every query taking more than 300ms will be logged. We do not recommend logging all the queries, because this will cause a lot of noise, and it will slow down both the system and your optimizing work. That’s why 300ms is generally a good start. But if you have very slow queries, you could use 500ms or even 1s.

Why do we use PG _ Stat _ statements in PostgreSQL?

The idea behind pg_stat_statements is to group identical queries, which are just used with different parameters and aggregate runtime information in a system view. In my personal judgement pg_stat_statements is really like a swiss army knife. It allows you to understand, what is really going on on your system.

When to reread PostgreSQL run time configuration?

The configuration file is reread whenever the postmaster process receives a SIGHUP signal (which is most easily sent by means of pg_ctl reload ). The postmaster also propagates this signal to all currently running server processes so that existing sessions also get the new value.