From: Tom Lane <tgl@sss.pgh.pa.us>
To: Edmund Horner <ejrh00@gmail.com>
cc: David Rowley <david.rowley@2ndquadrant.com>, tomas.vondra@2ndquadrant.com,
        PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Tid scan improvements
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Comments: In-reply-to Edmund Horner <ejrh00@gmail.com>
	message dated "Tue, 27 Nov 2018 21:16:46 +1300"
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Date: Fri, 21 Dec 2018 13:10:52 -0500
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BTW, with respect to this bit in 0001:

@@ -1795,6 +1847,15 @@ nulltestsel(PlannerInfo *root, NullTestType nulltes=
ttype, Node *arg,
                 return (Selectivity) 0; /* keep compiler quiet */
         }
     }
+    else if (vardata.var && IsA(vardata.var, Var) &&
+             ((Var *) vardata.var)->varattno =3D=3D SelfItemPointerAttrib=
uteNumber)
+    {
+        /*
+         * There are no stats for system columns, but we know CTID is nev=
er
+         * NULL.
+         */
+        selec =3D (nulltesttype =3D=3D IS_NULL) ? 0.0 : 1.0;
+    }
     else
     {
         /*

I'm not entirely sure why you're bothering; surely nulltestsel is
unrelated to what this patch is about?  And would anybody really
write "WHERE ctid IS NULL"?

However, if we do think it's worth adding code to cover this case,
I wouldn't make it specific to CTID.  *All* system columns can be
assumed not null, see heap_getsysattr().

			regards, tom lane