Suggest to the user the column they may have meant to reference.

Error messages informing the user that no such column exists can
sometimes provoke a perplexed response.  This often happens due to
a subtle typo in the column name or, perhaps less likely, in the
alias name.  To speed discovery of what the real issue is in such
cases, we'll now search the range table for approximate matches.
If there are one or two such matches that are good enough to think
that they might be what the user intended to type, and better than
all other approximate matches, we'll issue a hint suggesting that
the user might have intended to reference those columns.

Peter Geoghegan and Robert Haas

src/include/parser/parse_relation.h

@@ -16,6 +16,24 @@
 
 #include "parser/parse_node.h"
 
+
+/*
+ * Support for fuzzily matching column.
+ *
+ * This is for building diagnostic messages, where non-exact matching
+ * attributes are suggested to the user.  The struct's fields may be facets of
+ * a particular RTE, or of an entire range table, depending on context.
+ */
+typedef struct
+{
+	int				distance;	/* Weighted distance (lowest so far) */
+	RangeTblEntry  *rfirst;		/* RTE of first */
+	AttrNumber		first;		/* Closest attribute so far */
+	RangeTblEntry  *rsecond;	/* RTE of second */
+	AttrNumber		second;		/* Second closest attribute so far */
+} FuzzyAttrMatchState;
+
+
 extern RangeTblEntry *refnameRangeTblEntry(ParseState *pstate,
 					 const char *schemaname,
 					 const char *refname,
@@ -35,7 +53,8 @@ extern RangeTblEntry *GetRTEByRangeTablePosn(ParseState *pstate,
 extern CommonTableExpr *GetCTEForRTE(ParseState *pstate, RangeTblEntry *rte,
 			 int rtelevelsup);
 extern Node *scanRTEForColumn(ParseState *pstate, RangeTblEntry *rte,
-				 char *colname, int location);
+				 char *colname, int location,
+				 int fuzzy_rte_penalty, FuzzyAttrMatchState *fuzzystate);
 extern Node *colNameToVar(ParseState *pstate, char *colname, bool localonly,
 			 int location);
 extern void markVarForSelectPriv(ParseState *pstate, Var *var,