How to determine RAM usage of a query

Question

What are the factors that would affect the RAM usage by a query?

How do I assess the rough RAM usage of a query given the complete SQL?

This is regarding the MySQL RDBMS.

Answer 1

MySQL uses RAM in many ways...

• For caching data and indexes. In some sense, it does not matter how much RAM, since it is cache. This may be the biggest use, but it is a "shared" use.

• A complex SELECT may need a temporary table (eg, for GROUP BY and/or ORDER BY and/or subquery). This may be done in RAM, or it may not even attempt to do it in RAM, but instead do it on disk. There is one setting that puts a limit on the size of each in-memory temp table.

• The parsing of the statement takes a little bit of RAM.

• The binlog is buffered through RAM.

• "Group commit" is buffered in RAM.

• Galera does its "will this commit with a deadlock" logic in RAM.

• InnoDB uses bits to keep track of what is locked.

Shall I go on? Your question is not easy to answer. Furthermore, I would argue that it is mostly irrelevant. Would you care to explain why you asked?