From the example below and looking at scan.c, it seems that nested
block comments are supported.
test=# select /* /* nested */ */ 1;
?column?
----------
1
(1 row)
From section 35.3:
There are two types of comments in PL/pgSQL. A double dash (--) starts
a comment that extends to the end of the line. A /* starts a block
comment that extends to the next occurrence of */. Block comments
cannot be nested, but double dash comments can be enclosed into a
block comment and a double dash can hide the block comment delimiters
/* and */.
