Many stdlib functions / operations can't get inlined by postgres

[msullivan]

json -> scalar casts and array indexing are two culprits that showed up in some recent testing I've done, and together slowed a test down by about 2x.

The culprit seems to be this rule from https://wiki.postgresql.org/wiki/Inlining_of_SQL_functions:

• if the function is declared IMMUTABLE, then the expression must not invoke any non-immutable function or operator

[msullivan]

We can't just mark _index and jsonb_extract_scalar as being stable, though, since they might be used from indexes/constraints.
Options I've thought of so far:

• Create both stable and immutable versions of functions like this, call based on context
• Use our trampolines as the immutable versions, make the baseline ones stable.
• Inline the function bodies ourselves

[1st1]

Use our trampolines as the immutable versions, make the baseline ones stable.

Isn't this the best option?

[msullivan]

Use our trampolines as the immutable versions, make the baseline ones stable.

Isn't this the best option?

Yes, but I thought of it last, after I had already started typing a list :P