TODO Item - Return compressed length of TOAST datatypes (WIP)

I thought I would have a look at:

(Datatypes) Add function to return compressed length of TOAST data values.

A WIP patch is attached for comment (wanted to check I hadn't bitten off
more than I could chew *before* asking questions!).

A few questions come to mind:

1) The name - I have called it 'toast_compressed_length'. Seems longish
- I'm wondering if just 'compressed_length' is ok?

2) What should be returned for toasted data that is not compressed (or
plain stored data for that matter)? The WIP patch just gives the
uncompressed size (I notice I may need to subtract VARHDRSZ in some cases).

3) What should be returned for non-varlena types? The WIP patch is
treating everything as a varlena, so is returning incorrect information
for that case.

4) The builtin is declared as immutable - I am not so sure about that (I
am wondering if altering a column's storage from MAIN -> EXTENDED and
then updating the column to be itself will fool it).

5) Any multi-byte locale considerations?

regards

Mark