Here are some documentation patches about inlining SQL-language functions. Postgres has been able to inline both scalar and set-returning functions since the 9.x days (as long as they are LANGUAGE SQL and meet a bunch of other conditions). But this was never documented outside of a wiki page[1]. This is a very significant optimization, especially for SRFs with qual pushdown, but it is largely unknown. I recently checked five books on advanced Postgres usage, some specifically on query optimization, and none of them mentioned this feature. I think we should talk about it.
Putting this in our docs also gives us a useful reference point for some support requests that do similar inlining, but with more user control. We already have a support request to let users inline single-result SQL functions[2], and I have a patch to do something similar for set-returning SQL functions[3]. I gave a talk at Postgres Extensions Day in Montreal that ties all this together,[4] and our docs could do something similar.
The first patch just adds <sect2> elements to break up the Function Optimization section into two sub-sections: one covering declarative annotations and another covering support functions. No rewriting is needed.
The second patch adds a new <sect2> explaining how we inline SQL functions: both single-result and set-returning. Since this happens automatically, it makes a nice progression with the (easy) declarative annotations and the (hard) support functions.
I wonder if we should have some tests about this behavior also? I'm happy to add those, either as part of this commitfest entry or something separate.
