MIME-Version: 1.0
References: <202401181146.fuoeskfzriq7@alvherre.pgsql>
 <202401181711.qxjxpnl3ohnw@alvherre.pgsql>
In-Reply-To: <202401181711.qxjxpnl3ohnw@alvherre.pgsql>
From: Amit Langote <amitlangote09@gmail.com>
Date: Fri, 19 Jan 2024 12:09:49 +0900
Message-ID: 
 <CA+HiwqEZP-ybbvTZ_2gKJD2p60hvDkL9-UsoKrzirZkwdV60Ng@mail.gmail.com>
Subject: Re: remaining sql/json patches
To: Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: Andres Freund <andres@anarazel.de>, Erik Rijkers <er@xs4all.nl>,
	jian he <jian.universality@gmail.com>, Andrew Dunstan <andrew@dunslane.net>,
	PostgreSQL-development <pgsql-hackers@postgresql.org>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Archived-At: 
 <https://www.postgresql.org/message-id/CA%2BHiwqEZP-ybbvTZ_2gKJD2p60hvDkL9-UsoKrzirZkwdV60Ng%40mail.gmail.com>
Precedence: bulk

On Fri, Jan 19, 2024 at 2:11=E2=80=AFAM Alvaro Herrera <alvherre@alvh.no-ip=
.org> wrote:
> On 2024-Jan-18, Alvaro Herrera wrote:
>
> > commands/explain.c (Hmm, I think this is a preexisting bug actually)
> >
> >     3893          18 :         case T_TableFuncScan:
> >     3894          18 :             Assert(rte->rtekind =3D=3D RTE_TABLE=
FUNC);
> >     3895          18 :             if (rte->tablefunc)
> >     3896           0 :                 if (rte->tablefunc->functype =3D=
=3D TFT_XMLTABLE)
> >     3897           0 :                     objectname =3D "xmltable";
> >     3898             :                 else            /* Must be TFT_J=
SON_TABLE */
> >     3899           0 :                     objectname =3D "json_table";
> >     3900             :             else
> >     3901          18 :                 objectname =3D NULL;
> >     3902          18 :             objecttag =3D "Table Function Name";
> >     3903          18 :             break;
>
> Indeed -- the problem seems to be that add_rte_to_flat_rtable is
> creating a new RTE and zaps the ->tablefunc pointer for it.  So when
> EXPLAIN goes to examine the struct, there's a NULL pointer there and
> nothing is printed.

Ah yes.

> One simple fix is to change add_rte_to_flat_rtable so that it doesn't
> zero out the tablefunc pointer, but this is straight against what that
> function is trying to do, namely to remove substructure.

Yes.

>  Which means
> that we need to preserve the name somewhere else.  I added a new member
> to RangeTblEntry for this, which perhaps is a little ugly.  So here's
> the patch for that.
>
>  (I also added an alias to one XMLTABLE invocation
> under EXPLAIN, to show what it looks like when an alias is specified.
> Otherwise they're always shown as "XMLTABLE" "xmltable" which is a bit
> dumb).

Thanks for the patch.  Seems alright to me.

> Another possible way out is to decide that we don't want the
> "objectname" to be reported here.  I admit it's perhaps redundant.  In
> this case we'd just remove lines 3896-3899 shown above and let it be
> NULL.

Showing the function's name spelled out in the query (XMLTABLE /
JSON_TABLE) seems fine to me, even though maybe a bit redundant, yes.

--=20
Thanks, Amit Langote
EDB: http://www.enterprisedb.com