Revert: Remove useless self-joins

This commit reverts d3d55ce and subsequent fixes 2b26a69, 93c85db,
b44a170, b7f315c, 8a8ed91, b5fb673, 0a93f80, e047783,
a7928a5, 5ef34a8, 30b4955, 8c441c0, 028b154, fe09399,
489072a, and 466979e.

We are quite late in the release cycle and new bugs continue to appear.  Even
though we have fixes for all known bugs, there is a risk of throwing many
bugs to end users.

The plan for self-join elimination would be to do more review and testing,
then re-commit in the early v18 cycle.

Reported-by: Tom Lane
Discussion: https://postgr.es/m/2422119.1714691974%40sss.pgh.pa.us

Author: akorotkov

doc/src/sgml/config.sgml

@@ -5586,22 +5586,6 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
       </listitem>
      </varlistentry>
 
-     <varlistentry id="guc-enable_self_join_removal" xreflabel="enable_self_join_removal">
-      <term><varname>enable_self_join_removal</varname> (<type>boolean</type>)
-      <indexterm>
-       <primary><varname>enable_self_join_removal</varname> configuration parameter</primary>
-      </indexterm>
-      </term>
-      <listitem>
-       <para>
-        Enables or disables the query planner's optimization which analyses
-        the query tree and replaces self joins with semantically equivalent
-        single scans.  Takes into consideration only plain tables.
-        The default is <literal>on</literal>.
-       </para>
-      </listitem>
-     </varlistentry>
-
      <varlistentry id="guc-enable-seqscan" xreflabel="enable_seqscan">
       <term><varname>enable_seqscan</varname> (<type>boolean</type>)
       <indexterm>

src/backend/optimizer/path/indxpath.c

@@ -3440,22 +3440,6 @@ bool
 relation_has_unique_index_for(PlannerInfo *root, RelOptInfo *rel,
 							  List *restrictlist,
 							  List *exprlist, List *oprlist)
-{
-	return relation_has_unique_index_ext(root, rel, restrictlist,
-										 exprlist, oprlist, NULL);
-}
-
-/*
- * relation_has_unique_index_ext
- *	  Same as relation_has_unique_index_for(), but supports extra_clauses
- *	  parameter.  If extra_clauses isn't NULL, return baserestrictinfo clauses
- *	  which were used to derive uniqueness.
- */
-bool
-relation_has_unique_index_ext(PlannerInfo *root, RelOptInfo *rel,
-							  List *restrictlist,
-							  List *exprlist, List *oprlist,
-							  List **extra_clauses)
 {
 	ListCell   *ic;
 
@@ -3511,7 +3495,6 @@ relation_has_unique_index_ext(PlannerInfo *root, RelOptInfo *rel,
 	{
 		IndexOptInfo *ind = (IndexOptInfo *) lfirst(ic);
 		int			c;
-		List	   *exprs = NIL;
 
 		/*
 		 * If the index is not unique, or not immediately enforced, or if it's
@@ -3563,24 +3546,6 @@ relation_has_unique_index_ext(PlannerInfo *root, RelOptInfo *rel,
 				if (match_index_to_operand(rexpr, c, ind))
 				{
 					matched = true; /* column is unique */
-
-					if (bms_membership(rinfo->clause_relids) == BMS_SINGLETON)
-					{
-						MemoryContext oldMemCtx =
-							MemoryContextSwitchTo(root->planner_cxt);
-
-						/*
-						 * Add filter clause into a list allowing caller to
-						 * know if uniqueness have made not only by join
-						 * clauses.
-						 */
-						Assert(bms_is_empty(rinfo->left_relids) ||
-							   bms_is_empty(rinfo->right_relids));
-						if (extra_clauses)
-							exprs = lappend(exprs, rinfo);
-						MemoryContextSwitchTo(oldMemCtx);
-					}
-
 					break;
 				}
 			}
@@ -3623,11 +3588,7 @@ relation_has_unique_index_ext(PlannerInfo *root, RelOptInfo *rel,
 
 		/* Matched all key columns of this index? */
 		if (c == ind->nkeycolumns)
-		{
-			if (extra_clauses)
-				*extra_clauses = exprs;
 			return true;
-		}
 	}
 
 	return false;