Update docs on comparing execution plans when optimizing queries
What does this MR do?
importance of buffers has been a recurring topic in different database related discussion. @NikolayS is always very helpful in explaining this but I thought it may be good to add a paragraph about it in the docs.
This is basically a compilation from different replies/comments from Nikolay:
- !37786 (comment 399545400)
- !46289 (comment 440494402)
- https://gitlab.slack.com/archives/C3NBYFJ6N/p1600121920162900 (internal)
I hope a managed to summarize the gist of it in a way that make sense.