Jumble Query with COERCE_SQL_SYNTAX

Good day.

v14 introduced the way to get original text for some kind of expressions
using new 'funcformat' - COERCE_SQL_SYNTAX:
- EXTRACT(part from timestamp)
- (text IS [form] NORMALIZED)
and others.

Mentioned EXTRACT and NORMALIZED statements has parts, that are not
usual arguments but some kind of syntax. At least, there is no way to:

PREPARE a(text) as select extract($1 from now());

But JumbleExpr doesn't distinguish it and marks this argument as a
variable constant, ie remembers it in 'clocations'.

I believe such "non-variable constant" should not be jumbled as
replaceable thing.

In our case (extended pg_stat_statements), we attempt to generalize
plan and then explain generalized plan. But using constant list from
JumbleState we mistakenly replace first argument in EXTRACT expression
with parameter. And then 'get_func_sql_syntax' fails on assertion "first
argument is text constant".

Sure we could workaround in our plan mutator with skipping such first
argument. But I wonder, is it correct at all to not count it as a
non-modifiable syntax part in JumbleExpr?

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regards,

Sokolov Yura
y(dot)sokolov(at)postgrespro(dot)ru
funny(dot)falcon(at)gmail(dot)coma