Re: [HACKERS] patch: function xmltable

On 2017-01-24 21:32:56 -0300, Alvaro Herrera wrote:
> Andres Freund wrote:
> > Hi,
> > 
> > On 2017-01-24 17:38:49 -0300, Alvaro Herrera wrote:
> > > +static Datum ExecEvalTableExpr(TableExprState *tstate, ExprContext *econtext,
> > > +                  bool *isnull);
> > > +static Datum ExecEvalTableExprFast(TableExprState *exprstate, ExprContext *econtext,
> > > +                      bool *isNull);
> > > +static Datum tabexprFetchRow(TableExprState *tstate, ExprContext *econtext,
> > > +                bool *isNull);
> > > +static void tabexprInitialize(TableExprState *tstate, ExprContext *econtext,
> > > +                  Datum doc);
> > > +static void ShutdownTableExpr(Datum arg);
> > 
> > To me this (and a lot of the other code) hints quite strongly that
> > expression evalution is the wrong approach to implementing this.  What
> > you're essentially doing is building a vulcano style scan node.  Even if
> > we can this, we shouldn't double down on the bad decision to have these
> > magic expressions that return multiple rows.  There's historical reason
> > for tSRFs, but we shouldn't add more weirdness like this.
> 
> Thanks for giving it a look.  I have long thought that this patch would
> be at odds with your overall executor work.

Not fundamentally, but it makes it harder.


> XMLTABLE is specified by the standard to return multiple rows ... but
> then as far as my reading goes, it is only supposed to be supported in
> the range table (FROM clause) not in the target list.  I wonder if
> this would end up better if we only tried to support it in RT.  I asked
> Pavel to implement it like that a few weeks ago, but ...

Right - it makes sense in the FROM list - but then it should be an
executor node, instead of some expression thingy.

Greetings,

Andres Freund