Track more precisely query locations for nested statements

Previously, a Query generated through the transform phase would have
unset stmt_location, tracking the starting point of a query string.

Extensions relying on the statement location to extract its relevant
parts in the source text string would fallback to use the whole
statement instead, leading to confusing results like in
pg_stat_statements for queries relying on nested queries, like:
- EXPLAIN, with top-level and nested query using the same query string,
and a query ID coming from the nested query when the non-top-level
entry.
- Multi-statements, with only partial portions of queries being
normalized.
- COPY TO with a query, SELECT or DMLs.

This patch improves things by keeping track of the statement locations
and propagate it to Query during transform, allowing PGSS to only show
the relevant part of the query for nested query.  This leads to less
bloat in entries for non-top-level entries, as queries can now be
grouped within the same (toplevel, queryid) duos in pg_stat_statements.
The result gives a stricter one-one mapping between query IDs and its
query strings.

The regression tests introduced in 45e0ba30fc produce differences
reflecting the new logic.

Author: Anthonin Bonnefoy
Reviewed-by: Michael Paquier, Jian He
Discussion: https://postgr.es/m/CAO6_XqqM6S9bQ2qd=75W+yKATwoazxSNhv5sjW06fjGAtHbTUA@mail.gmail.com

src/backend/parser/analyze.c

@@ -238,24 +238,108 @@ parse_sub_analyze(Node *parseTree, ParseState *parentParseState,
 	return query;
 }
 
+/*
+ * setQueryLocationAndLength
+ * 		Set query's location and length from statement and ParseState
+ *
+ * Some statements, like PreparableStmt, can be located within parentheses.
+ * For example "(SELECT 1)" or "COPY (UPDATE ...) to x;".  For those, we
+ * cannot use the whole string from the statement's location or the SQL
+ * string would yield incorrectly.  The parser will set stmt_len, reflecting
+ * the size of the statement within the parentheses.  Thus, when stmt_len is
+ * available, we need to use it for the Query's stmt_len.
+ *
+ * For other cases, the parser can't provide the length of individual
+ * statements.  However, we have the statement's location plus the length
+ * (p_stmt_len) and location (p_stmt_location) of the top level RawStmt,
+ * stored in pstate.  Thus, the statement's length is the RawStmt's length
+ * minus how much we've advanced in the RawStmt's string.
+ */
+static void
+setQueryLocationAndLength(ParseState *pstate, Query *qry, Node *parseTree)
+{
+	ParseLoc	stmt_len = 0;
+
+	/*
+	 * If there is no information about the top RawStmt's length, leave it at
+	 * 0 to use the whole string.
+	 */
+	if (pstate->p_stmt_len == 0)
+		return;
+
+	switch (nodeTag(parseTree))
+	{
+		case T_InsertStmt:
+			qry->stmt_location = ((InsertStmt *) parseTree)->stmt_location;
+			stmt_len = ((InsertStmt *) parseTree)->stmt_len;
+			break;
+
+		case T_DeleteStmt:
+			qry->stmt_location = ((DeleteStmt *) parseTree)->stmt_location;
+			stmt_len = ((DeleteStmt *) parseTree)->stmt_len;
+			break;
+
+		case T_UpdateStmt:
+			qry->stmt_location = ((UpdateStmt *) parseTree)->stmt_location;
+			stmt_len = ((UpdateStmt *) parseTree)->stmt_len;
+			break;
+
+		case T_MergeStmt:
+			qry->stmt_location = ((MergeStmt *) parseTree)->stmt_location;
+			stmt_len = ((MergeStmt *) parseTree)->stmt_len;
+			break;
+
+		case T_SelectStmt:
+			qry->stmt_location = ((SelectStmt *) parseTree)->stmt_location;
+			stmt_len = ((SelectStmt *) parseTree)->stmt_len;
+			break;
+
+		case T_PLAssignStmt:
+			qry->stmt_location = ((PLAssignStmt *) parseTree)->location;
+			break;
+
+		default:
+			qry->stmt_location = pstate->p_stmt_location;
+			break;
+	}
+
+	if (stmt_len > 0)
+	{
+		/* Statement's length is known, use it */
+		qry->stmt_len = stmt_len;
+	}
+	else
+	{
+		/*
+		 * Compute the statement's length from the statement's location and
+		 * the RawStmt's length and location.
+		 */
+		qry->stmt_len = pstate->p_stmt_len - (qry->stmt_location - pstate->p_stmt_location);
+	}
+
+	/* The calculated statement length should be calculated as positive. */
+	Assert(qry->stmt_len >= 0);
+}
+
 /*
  * transformTopLevelStmt -
  *	  transform a Parse tree into a Query tree.
  *
- * This function is just responsible for transferring statement location data
- * from the RawStmt into the finished Query.
+ * This function is just responsible for storing location data
+ * from the RawStmt into the ParseState.
  */
 Query *
 transformTopLevelStmt(ParseState *pstate, RawStmt *parseTree)
 {
 	Query	   *result;
 
+	/* Store RawStmt's length and location in pstate */
+	pstate->p_stmt_len = parseTree->stmt_len;
+	pstate->p_stmt_location = parseTree->stmt_location;
+
 	/* We're at top level, so allow SELECT INTO */
 	result = transformOptionalSelectInto(pstate, parseTree->stmt);
 
-	result->stmt_location = parseTree->stmt_location;
-	result->stmt_len = parseTree->stmt_len;
-
 	return result;
 }
 
@@ -424,6 +508,7 @@ transformStmt(ParseState *pstate, Node *parseTree)
 	/* Mark as original query until we learn differently */
 	result->querySource = QSRC_ORIGINAL;
 	result->canSetTag = true;
+	setQueryLocationAndLength(pstate, result, parseTree);
 
 	return result;
 }