fix(clickhouse): Remove CURRENT_TIMESTAMP from NO_PAREN_FUNCTIONS

[VaggelisD]

Fixes #4076

[hellozepp]

@VaggelisD Can we add quoted arg to CURRENT_TIMESTAMP in ClickHouse?

e.g: this=Identifier(this=current_timestamp, quoted=True))

This will ensure that the target dialect will not have ambiguity when transpiling whether it is a field or a sql expression?

I encountered this problem when transpiling to spark, which supports this sql expression. Although it is not easy to handle, I did it like this:

def _escape_current_timestamp(expression: exp.Select):
    for column in expression.find_all(exp.Column):
        if column.this.name.upper() == 'CURRENT_TIMESTAMP':
            column.this.set("quoted", True)

Is this right?

[georgesittas]

This isn't the correct way to do it, you need to add RESERVED_KEYWORDS for spark or whatever you want to transpile towards.

[hellozepp]

@georgesittas Tks, this works for me.

My understanding is that when current_timestamp is used as a sql expression in the source dialect, it will be parsed as expressions=[CurrentTimestamp()], so it does not affect our RESERVED of current_timestamp. Right?

[georgesittas]

If you're parsing using clickhouse that's not right, for example:

>>> import sqlglot
>>> sqlglot.parse_one("current_timestamp", "clickhouse")
Column(
  this=Identifier(this=current_timestamp, quoted=False))

This would be quoted if current_timestamp is among the reserved expressions.