Re: patch: function xmltable

From: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
To: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
Cc: Craig Ringer <craig(at)2ndquadrant(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: patch: function xmltable
Date: 2016-11-28 22:34:33
Message-ID: 20161128223433.mm55e6altkso3qx6@alvherre.pgsql
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Pavel Stehule wrote:

> Here is updated patch without default namespace support (and without XPath
> expression transformation).
>
> Due last changes in parser
> https://github.com/postgres/postgres/commit/906bfcad7ba7cb3863fe0e2a7810be8e3cd84fbd
> I had to use c_expr on other positions ( xmlnamespace definition).
>
> I don't think it is limit - in 99% there will be const literal.

Argh. I can't avoid the feeling that I'm missing some parser trickery
here. We have the XMLNAMESPACES keyword and the clause-terminating
comma to protect these clauses, there must be a way to define this piece
of the grammar so that there's no conflict, without losing the freedom
in the expressions. But I don't see how. Now I agree that xml
namespace definitions are going to be string literals in almost all
cases (or in extra sophisticated cases, column names) ... it's probably
better to spend the bison-fu in the document expression or the column
options, or better yet the xmlexists_argument stuff. But I don't see
possibility of improvements in any of those places, so let's put it
aside -- we can improve later, if need arises.

In any case, it looks like we can change c_expr to b_expr in a few
places, which is good because then operators work (in particular, unless
I misread the grammar, foo||bar doesn't work with c_expr and does work
with b_expr, which seems the most useful in this case). Also, it makes
no sense to support (in the namespaces clause) DEFAULT a_expr if the
IDENT case uses only b_expr, so let's reduce both to just b_expr.

While I'm looking at node definitions, I see a few things that could use
some naming improvement. For example, "expr" for TableExpr is a bit
unexpressive. We could use "document_expr" there, perhaps. "row_path"
seems fixated on the XML case and the expression be path; let's use
"row_expr" there. And "cols" could be "column_exprs" perhaps. (All
those renames cause fall-out in various node-related files, so let's
think carefully to avoid renaming them multiple times.)

In primnodes, you kept the comment that says "xpath". Please update
that to not-just-XML reality.

Please fix the comment in XmlTableAddNs; NULL is no longer a valid value.

parse_expr.c has two unused variables; please remove them.

This test in ExecEvalTableExprProtected looks weird:
if (i != tstate->for_ordinality_col - 1)
please change to comparing "i + 1" (convert array index into attribute
number), and invert the boolean expression, leaving the for_ordinality
case on top and the rest in the "else". That seems easier to read.
Also, we customarily use post-increment (rownum++) instead of pre-incr.

In execQual.c I think it's neater to have ExecEvalTableExpr go before
its subroutine. Actually, I wonder whether it is really necessary to
have a subroutine in the first place; you could just move the entire
contents of that subroutine to within the PG_TRY block instead. The
only thing you lose is one indentation level. I'm not sure about this
one, but it's worth considering.

--
Álvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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