From 0b03e5951bf0a1a8868db13f02049cf686a82165 Mon Sep 17 00:00:00 2001
From: Robert Haas <rhaas@postgresql.org>
Date: Fri, 7 Nov 2014 17:26:02 -0500
Subject: Introduce custom path and scan providers.
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This allows extension modules to define their own methods for
scanning a relation, and get the core code to use them.  It's
unclear as yet how much use this capability will find, but we
won't find out if we never commit it.

KaiGai Kohei, reviewed at various times and in various levels
of detail by Shigeru Hanada, Tom Lane, Andres Freund, Álvaro
Herrera, and myself.
---
 src/backend/utils/adt/ruleutils.c | 70 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 70 insertions(+)

(limited to 'src/backend/utils/adt/ruleutils.c')

diff --git a/src/backend/utils/adt/ruleutils.c b/src/backend/utils/adt/ruleutils.c
index 24ade6cc201..bf4e81f554a 100644
--- a/src/backend/utils/adt/ruleutils.c
+++ b/src/backend/utils/adt/ruleutils.c
@@ -5493,6 +5493,26 @@ get_utility_query_def(Query *query, deparse_context *context)
 	}
 }
 
+/*
+ * GetSpecialCustomVar
+ *
+ * If a custom-scan provider uses a special varnode, this function will be
+ * called when deparsing; it should return an Expr node to be reversed-listed
+ * in lieu of the special Var.
+ */
+static Node *
+GetSpecialCustomVar(CustomScanState *css, Var *varnode, PlanState **child_ps)
+{
+	Assert(IsA(css, CustomScanState));
+	Assert(IS_SPECIAL_VARNO(varnode->varno));
+
+	if (!css->methods->GetSpecialCustomVar)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("%s does not support special varno reference",
+						css->methods->CustomName)));
+	return (Node *) css->methods->GetSpecialCustomVar(css, varnode, child_ps);
+}
 
 /*
  * Display a Var appropriately.
@@ -5522,6 +5542,8 @@ get_variable(Var *var, int levelsup, bool istoplevel, deparse_context *context)
 	int			netlevelsup;
 	deparse_namespace *dpns;
 	deparse_columns *colinfo;
+	PlanState  *child_ps = NULL;
+	Node	   *expr;
 	char	   *refname;
 	char	   *attname;
 
@@ -5546,6 +5568,29 @@ get_variable(Var *var, int levelsup, bool istoplevel, deparse_context *context)
 		colinfo = deparse_columns_fetch(var->varno, dpns);
 		attnum = var->varattno;
 	}
+	else if (IS_SPECIAL_VARNO(var->varno) &&
+			 IsA(dpns->planstate, CustomScanState) &&
+			 (expr = GetSpecialCustomVar((CustomScanState *) dpns->planstate,
+										 var, &child_ps)) != NULL)
+	{
+		deparse_namespace	save_dpns;
+
+		if (child_ps)
+			push_child_plan(dpns, child_ps, &save_dpns);
+		/*
+		 * Force parentheses because our caller probably assumed a Var is a
+		 * simple expression.
+		 */
+		if (!IsA(expr, Var))
+			appendStringInfoChar(buf, '(');
+		get_rule_expr((Node *) expr, context, true);
+		if (!IsA(expr, Var))
+			appendStringInfoChar(buf, ')');
+
+		if (child_ps)
+			pop_child_plan(dpns, &save_dpns);
+		return NULL;
+	}
 	else if (var->varno == OUTER_VAR && dpns->outer_tlist)
 	{
 		TargetEntry *tle;
@@ -5760,6 +5805,7 @@ get_name_for_var_field(Var *var, int fieldno,
 	AttrNumber	attnum;
 	int			netlevelsup;
 	deparse_namespace *dpns;
+	PlanState  *child_ps = NULL;
 	TupleDesc	tupleDesc;
 	Node	   *expr;
 
@@ -5834,6 +5880,30 @@ get_name_for_var_field(Var *var, int fieldno,
 		rte = rt_fetch(var->varno, dpns->rtable);
 		attnum = var->varattno;
 	}
+	else if (IS_SPECIAL_VARNO(var->varno) &&
+			 IsA(dpns->planstate, CustomScanState) &&
+			 (expr = GetSpecialCustomVar((CustomScanState *) dpns->planstate,
+										 var, &child_ps)) != NULL)
+	{
+		StringInfo		saved = context->buf;
+		StringInfoData	temp;
+		deparse_namespace save_dpns;
+
+		initStringInfo(&temp);
+		context->buf = &temp;
+
+		if (child_ps)
+			push_child_plan(dpns, child_ps, &save_dpns);
+		if (!IsA(expr, Var))
+			appendStringInfoChar(context->buf, '(');
+		get_rule_expr((Node *) expr, context, true);
+		if (!IsA(expr, Var))
+			appendStringInfoChar(context->buf, ')');
+		if (child_ps)
+			pop_child_plan(dpns, &save_dpns);
+		context->buf = saved;
+		return temp.data;
+	}
 	else if (var->varno == OUTER_VAR && dpns->outer_tlist)
 	{
 		TargetEntry *tle;
-- 
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