Expand virtual generated columns in the planner

Commit 83ea6c5 added support for virtual generated columns that are
computed on read.  All Var nodes in the query that reference virtual
generated columns must be replaced with the corresponding generation
expressions.  Currently, this replacement occurs in the rewriter.
However, this approach has several issues.  If a Var referencing a
virtual generated column has any varnullingrels, those varnullingrels
need to be propagated into the generation expression.  Failing to do
so can lead to "wrong varnullingrels" errors and improper outer-join
removal.

Additionally, if such a Var comes from the nullable side of an outer
join, we may need to wrap the generation expression in a
PlaceHolderVar to ensure that it is evaluated at the right place and
hence is forced to null when the outer join should do so.  In certain
cases, such as when the query uses grouping sets, we also need a
PlaceHolderVar for anything that is not a simple Var to isolate
subexpressions.  Failure to do so can result in incorrect results.

To fix these issues, this patch expands the virtual generated columns
in the planner rather than in the rewriter, and leverages the
pullup_replace_vars architecture to avoid code duplication.  The
generation expressions will be correctly marked with nullingrel bits
and wrapped in PlaceHolderVars when needed by the pullup_replace_vars
callback function.  This requires handling the OLD/NEW RETURNING list
Vars in pullup_replace_vars_callback, as it may now deal with Vars
referencing the result relation instead of a subquery.

The "wrong varnullingrels" error was reported by Alexander Lakhin.
The incorrect result issue and the improper outer-join removal issue
were reported by Richard Guo.

Author: Richard Guo <guofenglinux@gmail.com>
Author: Dean Rasheed <dean.a.rasheed@gmail.com>
Reviewed-by: Jian He <jian.universality@gmail.com>
Discussion: https://postgr.es/m/75eb1a6f-d59f-42e6-8a78-124ee808cda7@gmail.com

Author: Richard Guo

src/backend/optimizer/plan/planner.c

@@ -734,6 +734,14 @@ subquery_planner(PlannerGlobal *glob, Query *parse, PlannerInfo *parent_root,
 	 */
 	preprocess_function_rtes(root);
 
+	/*
+	 * Scan the rangetable for relations with virtual generated columns, and
+	 * replace all Var nodes in the query that reference these columns with
+	 * the generation expressions.  Recursion issues here are handled in the
+	 * same way as for SubLinks.
+	 */
+	parse = root->parse = expand_virtual_generated_columns(root);
+
 	/*
 	 * Check to see if any subqueries in the jointree can be merged into this
 	 * query.

src/backend/optimizer/prep/prepjointree.c

@@ -7,6 +7,7 @@
  *		replace_empty_jointree
  *		pull_up_sublinks
  *		preprocess_function_rtes
+ *		expand_virtual_generated_columns
  *		pull_up_subqueries
  *		flatten_simple_union_all
  *		do expression preprocessing (including flattening JOIN alias vars)
@@ -25,6 +26,7 @@
  */
 #include "postgres.h"
 
+#include "access/table.h"
 #include "catalog/pg_type.h"
 #include "funcapi.h"
 #include "miscadmin.h"
@@ -39,7 +41,9 @@
 #include "optimizer/tlist.h"
 #include "parser/parse_relation.h"
 #include "parser/parsetree.h"
+#include "rewrite/rewriteHandler.h"
 #include "rewrite/rewriteManip.h"
+#include "utils/rel.h"
 
 
 typedef struct nullingrel_info
@@ -58,6 +62,8 @@ typedef struct pullup_replace_vars_context
 	PlannerInfo *root;
 	List	   *targetlist;		/* tlist of subquery being pulled up */
 	RangeTblEntry *target_rte;	/* RTE of subquery */
+	int			result_relation;	/* the index of the result relation in the
+									 * rewritten query */
 	Relids		relids;			/* relids within subquery, as numbered after
 								 * pullup (set only if target_rte->lateral) */
 	nullingrel_info *nullinfo;	/* per-RTE nullingrel info (set only if
@@ -916,6 +922,133 @@ preprocess_function_rtes(PlannerInfo *root)
 	}
 }
 
+/*
+ * expand_virtual_generated_columns
+ *		Expand all virtual generated column references in a query.
+ *
+ * This scans the rangetable for relations with virtual generated columns, and
+ * replaces all Var nodes in the query that reference these columns with the
+ * generation expressions.  Note that we do not descend into subqueries; that
+ * is taken care of when the subqueries are planned.
+ *
+ * This has to be done after we have pulled up any SubLinks within the query's
+ * quals; otherwise any virtual generated column references within the SubLinks
+ * that should be transformed into joins wouldn't get expanded.
+ *
+ * Returns a modified copy of the query tree, if any relations with virtual
+ * generated columns are present.
+ */
+Query *
+expand_virtual_generated_columns(PlannerInfo *root)
+{
+	Query	   *parse = root->parse;
+	int			rt_index;
+	ListCell   *lc;
+
+	rt_index = 0;
+	foreach(lc, parse->rtable)
+	{
+		RangeTblEntry *rte = (RangeTblEntry *) lfirst(lc);
+		Relation	rel;
+		TupleDesc	tupdesc;
+
+		++rt_index;
+
+		/*
+		 * Only normal relations can have virtual generated columns.
+		 */
+		if (rte->rtekind != RTE_RELATION)
+			continue;
+
+		rel = table_open(rte->relid, NoLock);
+
+		tupdesc = RelationGetDescr(rel);
+		if (tupdesc->constr && tupdesc->constr->has_generated_virtual)
+		{
+			List	   *tlist = NIL;
+			pullup_replace_vars_context rvcontext;
+
+			for (int i = 0; i < tupdesc->natts; i++)
+			{
+				Form_pg_attribute attr = TupleDescAttr(tupdesc, i);
+				TargetEntry *tle;
+
+				if (attr->attgenerated == ATTRIBUTE_GENERATED_VIRTUAL)
+				{
+					Node	   *defexpr;
+
+					defexpr = build_generation_expression(rel, i + 1);
+					ChangeVarNodes(defexpr, 1, rt_index, 0);
+
+					tle = makeTargetEntry((Expr *) defexpr, i + 1, 0, false);
+					tlist = lappend(tlist, tle);
+				}
+				else
+				{
+					Var		   *var;
+
+					var = makeVar(rt_index,
+								  i + 1,
+								  attr->atttypid,
+								  attr->atttypmod,
+								  attr->attcollation,
+								  0);
+
+					tle = makeTargetEntry((Expr *) var, i + 1, 0, false);
+					tlist = lappend(tlist, tle);
+				}
+			}
+
+			Assert(list_length(tlist) > 0);
+			Assert(!rte->lateral);
+
+			/*
+			 * The relation's targetlist items are now in the appropriate form
+			 * to insert into the query, except that we may need to wrap them
+			 * in PlaceHolderVars.  Set up required context data for
+			 * pullup_replace_vars.
+			 */
+			rvcontext.root = root;
+			rvcontext.targetlist = tlist;
+			rvcontext.target_rte = rte;
+			rvcontext.result_relation = parse->resultRelation;
+			/* won't need these values */
+			rvcontext.relids = NULL;
+			rvcontext.nullinfo = NULL;
+			/* pass NULL for outer_hasSubLinks */
+			rvcontext.outer_hasSubLinks = NULL;
+			rvcontext.varno = rt_index;
+			/* this flag will be set below, if needed */
+			rvcontext.wrap_non_vars = false;
+			/* initialize cache array with indexes 0 .. length(tlist) */
+			rvcontext.rv_cache = palloc0((list_length(tlist) + 1) *
+										 sizeof(Node *));
+
+			/*
+			 * If the query uses grouping sets, we need a PlaceHolderVar for
+			 * anything that's not a simple Var.  Again, this ensures that
+			 * expressions retain their separate identity so that they will
+			 * match grouping set columns when appropriate.  (It'd be
+			 * sufficient to wrap values used in grouping set columns, and do
+			 * so only in non-aggregated portions of the tlist and havingQual,
+			 * but that would require a lot of infrastructure that
+			 * pullup_replace_vars hasn't currently got.)
+			 */
+			if (parse->groupingSets)
+				rvcontext.wrap_non_vars = true;
+
+			/*
+			 * Apply pullup variable replacement throughout the query tree.
+			 */
+			parse = (Query *) pullup_replace_vars((Node *) parse, &rvcontext);
+		}
+
+		table_close(rel, NoLock);
+	}
+
+	return parse;
+}
+
 /*
  * pull_up_subqueries
  *		Look for subqueries in the rangetable that can be pulled up into
@@ -1197,6 +1330,13 @@ pull_up_simple_subquery(PlannerInfo *root, Node *jtnode, RangeTblEntry *rte,
 	 */
 	preprocess_function_rtes(subroot);
 
+	/*
+	 * Scan the rangetable for relations with virtual generated columns, and
+	 * replace all Var nodes in the query that reference these columns with
+	 * the generation expressions.
+	 */
+	subquery = subroot->parse = expand_virtual_generated_columns(subroot);
+
 	/*
 	 * Recursively pull up the subquery's subqueries, so that
 	 * pull_up_subqueries' processing is complete for its jointree and
@@ -1274,6 +1414,7 @@ pull_up_simple_subquery(PlannerInfo *root, Node *jtnode, RangeTblEntry *rte,
 	rvcontext.root = root;
 	rvcontext.targetlist = subquery->targetList;
 	rvcontext.target_rte = rte;
+	rvcontext.result_relation = 0;
 	if (rte->lateral)
 	{
 		rvcontext.relids = get_relids_in_jointree((Node *) subquery->jointree,
@@ -1834,6 +1975,7 @@ pull_up_simple_values(PlannerInfo *root, Node *jtnode, RangeTblEntry *rte)
 	rvcontext.root = root;
 	rvcontext.targetlist = tlist;
 	rvcontext.target_rte = rte;
+	rvcontext.result_relation = 0;
 	rvcontext.relids = NULL;	/* can't be any lateral references here */
 	rvcontext.nullinfo = NULL;
 	rvcontext.outer_hasSubLinks = &parse->hasSubLinks;
@@ -1993,6 +2135,7 @@ pull_up_constant_function(PlannerInfo *root, Node *jtnode,
 													  NULL, /* resname */
 													  false));	/* resjunk */
 	rvcontext.target_rte = rte;
+	rvcontext.result_relation = 0;
 
 	/*
 	 * Since this function was reduced to a Const, it doesn't contain any
@@ -2490,6 +2633,10 @@ pullup_replace_vars_callback(Var *var,
 	bool		need_phv;
 	Node	   *newnode;
 
+	/* System columns are not replaced. */
+	if (varattno < InvalidAttrNumber)
+		return (Node *) copyObject(var);
+
 	/*
 	 * We need a PlaceHolderVar if the Var-to-be-replaced has nonempty
 	 * varnullingrels (unless we find below that the replacement expression is
@@ -2559,6 +2706,22 @@ pullup_replace_vars_callback(Var *var,
 		rowexpr->location = var->location;
 		newnode = (Node *) rowexpr;
 
+		/* Handle any OLD/NEW RETURNING list Vars */
+		if (var->varreturningtype != VAR_RETURNING_DEFAULT)
+		{
+			/*
+			 * Wrap the RowExpr in a ReturningExpr node, so that the executor
+			 * returns NULL if the OLD/NEW row does not exist.
+			 */
+			ReturningExpr *rexpr = makeNode(ReturningExpr);
+
+			rexpr->retlevelsup = 0;
+			rexpr->retold = (var->varreturningtype == VAR_RETURNING_OLD);
+			rexpr->retexpr = (Expr *) newnode;
+
+			newnode = (Node *) rexpr;
+		}
+
 		/*
 		 * Insert PlaceHolderVar if needed.  Notice that we are wrapping one
 		 * PlaceHolderVar around the whole RowExpr, rather than putting one
@@ -2588,6 +2751,39 @@ pullup_replace_vars_callback(Var *var,
 		/* Make a copy of the tlist item to return */
 		newnode = (Node *) copyObject(tle->expr);
 
+		/* Handle any OLD/NEW RETURNING list Vars */
+		if (var->varreturningtype != VAR_RETURNING_DEFAULT)
+		{
+			/*
+			 * Copy varreturningtype onto any Vars in the tlist item that
+			 * refer to result_relation (which had better be non-zero).
+			 */
+			if (rcon->result_relation == 0)
+				elog(ERROR, "variable returning old/new found outside RETURNING list");
+
+			SetVarReturningType((Node *) newnode, rcon->result_relation,
+								0, var->varreturningtype);
+
+			/*
+			 * If the replacement expression in the targetlist is not simply a
+			 * Var referencing result_relation, wrap it in a ReturningExpr
+			 * node, so that the executor returns NULL if the OLD/NEW row does
+			 * not exist.
+			 */
+			if (!IsA(newnode, Var) ||
+				((Var *) newnode)->varno != rcon->result_relation ||
+				((Var *) newnode)->varlevelsup != 0)
+			{
+				ReturningExpr *rexpr = makeNode(ReturningExpr);
+
+				rexpr->retlevelsup = 0;
+				rexpr->retold = (var->varreturningtype == VAR_RETURNING_OLD);
+				rexpr->retexpr = (Expr *) newnode;
+
+				newnode = (Node *) rexpr;
+			}
+		}
+
 		/* Insert PlaceHolderVar if needed */
 		if (need_phv)
 		{