Store tuples for EvalPlanQual in slots, rather than as HeapTuples.

For the upcoming pluggable table access methods it's quite
inconvenient to store tuples as HeapTuples, as that'd require
converting tuples from a their native format into HeapTuples. Instead
use slots to manage epq tuples.

To fit into that scheme, change the foreign data wrapper callback
RefetchForeignRow, to store the tuple in a slot. Insist on using the
caller provided slot, so it conveniently can be stored in the
corresponding EPQ slot.  As there is no in core user of
RefetchForeignRow, that change was done blindly, but we plan to test
that soon.

To avoid duplicating that work for row locks, move row locks to just
directly use the EPQ slots - it previously temporarily stored tuples
in LockRowsState.lr_curtuples, but that doesn't seem beneficial, given
we'd possibly end up with a significant number of additional slots.

The behaviour of es_epqTupleSet[rti -1] is now checked by
es_epqTupleSlot[rti -1] != NULL, as that is distinguishable from a
slot containing an empty tuple.

Author: Andres Freund, Haribabu Kommi, Ashutosh Bapat
Discussion: https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de

Author: anarazel

doc/src/sgml/fdwhandler.sgml

@@ -992,29 +992,31 @@ GetForeignRowMarkType(RangeTblEntry *rte,
 
     <para>
 <programlisting>
-HeapTuple
+void
 RefetchForeignRow(EState *estate,
                   ExecRowMark *erm,
                   Datum rowid,
+                  TupleTableSlot *slot,
                   bool *updated);
 </programlisting>
 
-     Re-fetch one tuple from the foreign table, after locking it if required.
+     Re-fetch one tuple slot from the foreign table, after locking it if required.
      <literal>estate</literal> is global execution state for the query.
      <literal>erm</literal> is the <structname>ExecRowMark</structname> struct describing
      the target foreign table and the row lock type (if any) to acquire.
      <literal>rowid</literal> identifies the tuple to be fetched.
-     <literal>updated</literal> is an output parameter.
+     <literal>slot</literal> contains nothing useful upon call, but can be used to
+     hold the returned tuple. <literal>updated</literal> is an output parameter.
     </para>
 
     <para>
-     This function should return a palloc'ed copy of the fetched tuple,
-     or <literal>NULL</literal> if the row lock couldn't be obtained.  The row lock
-     type to acquire is defined by <literal>erm-&gt;markType</literal>, which is the
-     value previously returned by <function>GetForeignRowMarkType</function>.
-     (<literal>ROW_MARK_REFERENCE</literal> means to just re-fetch the tuple without
-     acquiring any lock, and <literal>ROW_MARK_COPY</literal> will never be seen by
-     this routine.)
+     This function should store the tuple into the provided, or clear it if if
+     the row lock couldn't be obtained.  The row lock type to acquire is
+     defined by <literal>erm-&gt;markType</literal>, which is the value
+     previously returned by <function>GetForeignRowMarkType</function>.
+     (<literal>ROW_MARK_REFERENCE</literal> means to just re-fetch the tuple
+     without acquiring any lock, and <literal>ROW_MARK_COPY</literal> will
+     never be seen by this routine.)
     </para>
 
     <para>
@@ -1026,7 +1028,7 @@ RefetchForeignRow(EState *estate,
 
     <para>
      Note that by default, failure to acquire a row lock should result in
-     raising an error; a <literal>NULL</literal> return is only appropriate if
+     raising an error; returning with an empty slot is only appropriate if
      the <literal>SKIP LOCKED</literal> option is specified
      by <literal>erm-&gt;waitPolicy</literal>.
     </para>