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

Author: robertmhaas

src/backend/parser/parse_expr.c

@@ -556,7 +556,8 @@ transformColumnRef(ParseState *pstate, ColumnRef *cref)
 				colname = strVal(field2);
 
 				/* Try to identify as a column of the RTE */
-				node = scanRTEForColumn(pstate, rte, colname, cref->location);
+				node = scanRTEForColumn(pstate, rte, colname, cref->location,
+										0, NULL);
 				if (node == NULL)
 				{
 					/* Try it as a function call on the whole row */
@@ -601,7 +602,8 @@ transformColumnRef(ParseState *pstate, ColumnRef *cref)
 				colname = strVal(field3);
 
 				/* Try to identify as a column of the RTE */
-				node = scanRTEForColumn(pstate, rte, colname, cref->location);
+				node = scanRTEForColumn(pstate, rte, colname, cref->location,
+										0, NULL);
 				if (node == NULL)
 				{
 					/* Try it as a function call on the whole row */
@@ -659,7 +661,8 @@ transformColumnRef(ParseState *pstate, ColumnRef *cref)
 				colname = strVal(field4);
 
 				/* Try to identify as a column of the RTE */
-				node = scanRTEForColumn(pstate, rte, colname, cref->location);
+				node = scanRTEForColumn(pstate, rte, colname, cref->location,
+										0, NULL);
 				if (node == NULL)
 				{
 					/* Try it as a function call on the whole row */

src/backend/parser/parse_func.c

@@ -1779,7 +1779,7 @@ ParseComplexProjection(ParseState *pstate, char *funcname, Node *first_arg,
 									 ((Var *) first_arg)->varno,
 									 ((Var *) first_arg)->varlevelsup);
 		/* Return a Var if funcname matches a column, else NULL */
-		return scanRTEForColumn(pstate, rte, funcname, location);
+		return scanRTEForColumn(pstate, rte, funcname, location, 0, NULL);
 	}
 
 	/*

src/backend/parser/parse_relation.c

@@ -33,6 +33,8 @@
 #include "utils/syscache.h"
 
 
+#define MAX_FUZZY_DISTANCE				3
+
 static RangeTblEntry *scanNameSpaceForRefname(ParseState *pstate,
 						const char *refname, int location);
 static RangeTblEntry *scanNameSpaceForRelid(ParseState *pstate, Oid relid,
@@ -519,6 +521,101 @@ GetCTEForRTE(ParseState *pstate, RangeTblEntry *rte, int rtelevelsup)
 	return NULL;				/* keep compiler quiet */
 }
 
+/*
+ * updateFuzzyAttrMatchState
+ *	  Using Levenshtein distance, consider if column is best fuzzy match.
+ */
+static void
+updateFuzzyAttrMatchState(int fuzzy_rte_penalty,
+						  FuzzyAttrMatchState *fuzzystate, RangeTblEntry *rte,
+						  const char *actual, const char *match, int attnum)
+{
+	int		columndistance;
+	int		matchlen;
+
+	/* Bail before computing the Levenshtein distance if there's no hope. */
+	if (fuzzy_rte_penalty > fuzzystate->distance)
+		return;
+
+	/*
+	 * Outright reject dropped columns, which can appear here with apparent
+	 * empty actual names, per remarks within scanRTEForColumn().
+	 */
+	if (actual[0] == '\0')
+		return;
+
+	/* Use Levenshtein to compute match distance. */
+	matchlen = strlen(match);
+	columndistance =
+		varstr_levenshtein_less_equal(actual, strlen(actual), match, matchlen,
+									  1, 1, 1,
+									  fuzzystate->distance + 1
+										- fuzzy_rte_penalty);
+
+	/*
+	 * If more than half the characters are different, don't treat it as a
+	 * match, to avoid making ridiculous suggestions.
+	 */
+	if (columndistance > matchlen / 2)
+		return;
+
+	/*
+	 * From this point on, we can ignore the distinction between the
+	 * RTE-name distance and the column-name distance.
+	 */
+	columndistance += fuzzy_rte_penalty;
+
+	/*
+	 * If the new distance is less than or equal to that of the best match
+	 * found so far, update fuzzystate.
+	 */
+	if (columndistance < fuzzystate->distance)
+	{
+		/* Store new lowest observed distance for RTE */
+		fuzzystate->distance = columndistance;
+		fuzzystate->rfirst = rte;
+		fuzzystate->first = attnum;
+		fuzzystate->rsecond = NULL;
+		fuzzystate->second = InvalidAttrNumber;
+	}
+	else if (columndistance == fuzzystate->distance)
+	{
+		/*
+		 * This match distance may equal a prior match within this same
+		 * range table.  When that happens, the prior match may also be
+		 * given, but only if there is no more than two equally distant
+		 * matches from the RTE (in turn, our caller will only accept
+		 * two equally distant matches overall).
+		 */
+		if (AttributeNumberIsValid(fuzzystate->second))
+		{
+			/* Too many RTE-level matches */
+			fuzzystate->rfirst = NULL;
+			fuzzystate->first = InvalidAttrNumber;
+			fuzzystate->rsecond = NULL;
+			fuzzystate->second = InvalidAttrNumber;
+			/* Clearly, distance is too low a bar (for *any* RTE) */
+			fuzzystate->distance = columndistance - 1;
+		}
+		else if (AttributeNumberIsValid(fuzzystate->first))
+		{
+			/* Record as provisional second match for RTE */
+			fuzzystate->rsecond = rte;
+			fuzzystate->second = attnum;
+		}
+		else if (fuzzystate->distance <= MAX_FUZZY_DISTANCE)
+		{
+			/*
+			 * Record as provisional first match (this can occasionally
+			 * occur because previous lowest distance was "too low a
+			 * bar", rather than being associated with a real match)
+			 */
+			fuzzystate->rfirst = rte;
+			fuzzystate->first = attnum;
+		}
+	}
+}
+
 /*
  * scanRTEForColumn
  *	  Search the column names of a single RTE for the given name.
@@ -527,10 +624,14 @@ GetCTEForRTE(ParseState *pstate, RangeTblEntry *rte, int rtelevelsup)
  *
  * Side effect: if we find a match, mark the RTE as requiring read access
  * for the column.
+ *
+ * Additional side effect: if fuzzystate is non-NULL, check non-system columns
+ * for an approximate match and update fuzzystate accordingly.
  */
 Node *
 scanRTEForColumn(ParseState *pstate, RangeTblEntry *rte, char *colname,
-				 int location)
+				 int location, int fuzzy_rte_penalty,
+				 FuzzyAttrMatchState *fuzzystate)
 {
 	Node	   *result = NULL;
 	int			attnum = 0;
@@ -548,12 +649,16 @@ scanRTEForColumn(ParseState *pstate, RangeTblEntry *rte, char *colname,
 	 * Should this somehow go wrong and we try to access a dropped column,
 	 * we'll still catch it by virtue of the checks in
 	 * get_rte_attribute_type(), which is called by make_var().  That routine
-	 * has to do a cache lookup anyway, so the check there is cheap.
+	 * has to do a cache lookup anyway, so the check there is cheap.  Callers
+	 * interested in finding match with shortest distance need to defend
+	 * against this directly, though.
 	 */
 	foreach(c, rte->eref->colnames)
 	{
+		const char *attcolname = strVal(lfirst(c));
+
 		attnum++;
-		if (strcmp(strVal(lfirst(c)), colname) == 0)
+		if (strcmp(attcolname, colname) == 0)
 		{
 			if (result)
 				ereport(ERROR,
@@ -566,6 +671,11 @@ scanRTEForColumn(ParseState *pstate, RangeTblEntry *rte, char *colname,
 			markVarForSelectPriv(pstate, var, rte);
 			result = (Node *) var;
 		}
+
+		/* Updating fuzzy match state, if provided. */
+		if (fuzzystate != NULL)
+			updateFuzzyAttrMatchState(fuzzy_rte_penalty, fuzzystate,
+									  rte, attcolname, colname, attnum);
 	}
 
 	/*
@@ -642,7 +752,8 @@ colNameToVar(ParseState *pstate, char *colname, bool localonly,
 				continue;
 
 			/* use orig_pstate here to get the right sublevels_up */
-			newresult = scanRTEForColumn(orig_pstate, rte, colname, location);
+			newresult = scanRTEForColumn(orig_pstate, rte, colname, location,
+										 0, NULL);
 
 			if (newresult)
 			{
@@ -668,36 +779,92 @@ colNameToVar(ParseState *pstate, char *colname, bool localonly,
 
 /*
  * searchRangeTableForCol
- *	  See if any RangeTblEntry could possibly provide the given column name.
- *	  If so, return a pointer to the RangeTblEntry; else return NULL.
+ *	  See if any RangeTblEntry could possibly provide the given column name (or
+ *	  find the best match available).  Returns state with relevant details.
  *
  * This is different from colNameToVar in that it considers every entry in
  * the ParseState's rangetable(s), not only those that are currently visible
  * in the p_namespace list(s).  This behavior is invalid per the SQL spec,
  * and it may give ambiguous results (there might be multiple equally valid
  * matches, but only one will be returned).  This must be used ONLY as a
  * heuristic in giving suitable error messages.  See errorMissingColumn.
+ *
+ * This function is also different in that it will consider approximate
+ * matches -- if the user entered an alias/column pair that is only slightly
+ * different from a valid pair, we may be able to infer what they meant to
+ * type and provide a reasonable hint.
+ *
+ * The FuzzyAttrMatchState will have 'rfirst' pointing to the best RTE
+ * containing the most promising match for the alias and column name.  If
+ * the alias and column names match exactly, 'first' will be InvalidAttrNumber;
+ * otherwise, it will be the attribute number for the match.  In the latter
+ * case, 'rsecond' may point to a second, equally close approximate match,
+ * and 'second' will contain the attribute number for the second match.
  */
-static RangeTblEntry *
-searchRangeTableForCol(ParseState *pstate, char *colname, int location)
+static FuzzyAttrMatchState *
+searchRangeTableForCol(ParseState *pstate, const char *alias, char *colname,
+					   int location)
 {
 	ParseState *orig_pstate = pstate;
+	FuzzyAttrMatchState *fuzzystate = palloc(sizeof(FuzzyAttrMatchState));
+
+	fuzzystate->distance = MAX_FUZZY_DISTANCE + 1;
+	fuzzystate->rfirst = NULL;
+	fuzzystate->rsecond = NULL;
+	fuzzystate->first = InvalidAttrNumber;
+	fuzzystate->second = InvalidAttrNumber;
 
 	while (pstate != NULL)
 	{
 		ListCell   *l;
 
 		foreach(l, pstate->p_rtable)
 		{
-			RangeTblEntry *rte = (RangeTblEntry *) lfirst(l);
+			RangeTblEntry	   *rte = (RangeTblEntry *) lfirst(l);
+			int					fuzzy_rte_penalty = 0;
 
-			if (scanRTEForColumn(orig_pstate, rte, colname, location))
-				return rte;
+			/*
+			 * Typically, it is not useful to look for matches within join
+			 * RTEs; they effectively duplicate other RTEs for our purposes,
+			 * and if a match is chosen from a join RTE, an unhelpful alias is
+			 * displayed in the final diagnostic message.
+			 */
+			if (rte->rtekind == RTE_JOIN)
+				continue;
+
+			/*
+			 * If the user didn't specify an alias, then matches against one
+			 * RTE are as good as another.  But if the user did specify an
+			 * alias, then we want at least a fuzzy - and preferably an exact
+			 * - match for the range table entry.
+			 */
+			if (alias != NULL)
+				fuzzy_rte_penalty =
+					varstr_levenshtein(alias, strlen(alias),
+									   rte->eref->aliasname,
+									   strlen(rte->eref->aliasname),
+									   1, 1, 1);
+
+			/*
+			 * Scan for a matching column; if we find an exact match, we're
+			 * done.  Otherwise, update fuzzystate.
+			 */
+			if (scanRTEForColumn(orig_pstate, rte, colname, location,
+								 fuzzy_rte_penalty, fuzzystate)
+					&& fuzzy_rte_penalty == 0)
+			{
+				fuzzystate->rfirst = rte;
+				fuzzystate->first = InvalidAttrNumber;
+				fuzzystate->rsecond = NULL;
+				fuzzystate->second = InvalidAttrNumber;
+				return fuzzystate;
+			}
 		}
 
 		pstate = pstate->parentParseState;
 	}
-	return NULL;
+
+	return fuzzystate;
 }
 
 /*
@@ -2860,34 +3027,67 @@ void
 errorMissingColumn(ParseState *pstate,
 				   char *relname, char *colname, int location)
 {
-	RangeTblEntry *rte;
+	FuzzyAttrMatchState	   *state;
+	char				   *closestfirst = NULL;
 
 	/*
-	 * If relname was given, just play dumb and report it.  (In practice, a
-	 * bad qualification name should end up at errorMissingRTE, not here, so
-	 * no need to work hard on this case.)
+	 * Search the entire rtable looking for possible matches.  If we find one,
+	 * emit a hint about it.
+	 *
+	 * TODO: improve this code (and also errorMissingRTE) to mention using
+	 * LATERAL if appropriate.
 	 */
-	if (relname)
-		ereport(ERROR,
-				(errcode(ERRCODE_UNDEFINED_COLUMN),
-				 errmsg("column %s.%s does not exist", relname, colname),
-				 parser_errposition(pstate, location)));
+	state = searchRangeTableForCol(pstate, relname, colname, location);
 
 	/*
-	 * Otherwise, search the entire rtable looking for possible matches.  If
-	 * we find one, emit a hint about it.
+	 * Extract closest col string for best match, if any.
 	 *
-	 * TODO: improve this code (and also errorMissingRTE) to mention using
-	 * LATERAL if appropriate.
+	 * Infer an exact match referenced despite not being visible from the fact
+	 * that an attribute number was not present in state passed back -- this is
+	 * what is reported when !closestfirst.  There might also be an exact match
+	 * that was qualified with an incorrect alias, in which case closestfirst
+	 * will be set (so hint is the same as generic fuzzy case).
 	 */
-	rte = searchRangeTableForCol(pstate, colname, location);
-
-	ereport(ERROR,
-			(errcode(ERRCODE_UNDEFINED_COLUMN),
-			 errmsg("column \"%s\" does not exist", colname),
-			 rte ? errhint("There is a column named \"%s\" in table \"%s\", but it cannot be referenced from this part of the query.",
-						   colname, rte->eref->aliasname) : 0,
-			 parser_errposition(pstate, location)));
+	if (state->rfirst && AttributeNumberIsValid(state->first))
+		closestfirst = strVal(list_nth(state->rfirst->eref->colnames,
+									   state->first - 1));
+
+	if (!state->rsecond)
+	{
+		/*
+		 * Handle case where there is zero or one column suggestions to hint,
+		 * including exact matches referenced but not visible.
+		 */
+		ereport(ERROR,
+				(errcode(ERRCODE_UNDEFINED_COLUMN),
+				 relname ?
+				 errmsg("column %s.%s does not exist", relname, colname):
+				 errmsg("column \"%s\" does not exist", colname),
+				 state->rfirst ? closestfirst ?
+				 errhint("Perhaps you meant to reference the column \"%s\".\"%s\".",
+						 state->rfirst->eref->aliasname, closestfirst):
+				 errhint("There is a column named \"%s\" in table \"%s\", but it cannot be referenced from this part of the query.",
+						 colname, state->rfirst->eref->aliasname): 0,
+				 parser_errposition(pstate, location)));
+	}
+	else
+	{
+		/* Handle case where there are two equally useful column hints */
+		char				   *closestsecond;
+
+		closestsecond = strVal(list_nth(state->rsecond->eref->colnames,
+										state->second - 1));
+
+		ereport(ERROR,
+				(errcode(ERRCODE_UNDEFINED_COLUMN),
+				 relname ?
+				 errmsg("column %s.%s does not exist", relname, colname):
+				 errmsg("column \"%s\" does not exist", colname),
+				 errhint("Perhaps you meant to reference the column \"%s\".\"%s\" or the column \"%s\".\"%s\".",
+						 state->rfirst->eref->aliasname, closestfirst,
+						 state->rsecond->eref->aliasname, closestsecond),
+				 parser_errposition(pstate, location)));
+	}
 }