Adjust string comparison in jsonpath

We have implemented jsonpath string comparison using default database locale.
However, standard requires us to compare Unicode codepoints.  This commit
implements that, but for performance reasons we still use per-byte comparison
for "==" operator.  Thus, for consistency other comparison operators do per-byte
comparison if Unicode codepoints appear to be equal.

In some edge cases, when same Unicode codepoints have different binary
representations in database encoding, we diverge standard to achieve better
performance of "==" operator.  In future to implement strict standard
conformance, we can do normalization of input JSON strings.

Original patch was written by Nikita Glukhov, rewritten by me.

Reported-by: Markus Winand
Discussion: https://postgr.es/m/8B7FA3B4-328D-43D7-95A8-37B8891B8C78%40winand.at
Author: Nikita Glukhov, Alexander Korotkov
Backpatch-through: 12

src/backend/utils/adt/jsonpath_exec.c

@@ -1980,6 +1980,73 @@ executeComparison(JsonPathItem *cmp, JsonbValue *lv, JsonbValue *rv, void *p)
 	return compareItems(cmp->type, lv, rv);
 }
 
+/*
+ * Perform per-byte comparison of two strings.
+ */
+static int
+binaryCompareStrings(const char *s1, int len1,
+					 const char *s2, int len2)
+{
+	int			cmp;
+
+	cmp = memcmp(s1, s2, Min(len1, len2));
+
+	if (cmp != 0)
+		return cmp;
+
+	if (len1 == len2)
+		return 0;
+
+	return len1 < len2 ? -1 : 1;
+}
+
+/*
+ * Compare two strings in the current server encoding using Unicode codepoint
+ * collation.
+ */
+static int
+compareStrings(const char *mbstr1, int mblen1,
+			   const char *mbstr2, int mblen2)
+{
+	if (GetDatabaseEncoding() == PG_SQL_ASCII ||
+		GetDatabaseEncoding() == PG_UTF8)
+	{
+		/*
+		 * It's known property of UTF-8 strings that their per-byte comparison
+		 * result matches codepoints comparison result.  ASCII can be
+		 * considered as special case of UTF-8.
+		 */
+		return binaryCompareStrings(mbstr1, mblen1, mbstr2, mblen2);
+	}
+	else
+	{
+		/* We have to convert other encodings to UTF-8 first, then compare. */
+		char	   *utf8str1 = pg_server_to_any(mbstr1, mblen1, PG_UTF8),
+				   *utf8str2 = pg_server_to_any(mbstr2, mblen2, PG_UTF8);
+		int			cmp;
+
+		cmp = binaryCompareStrings(utf8str1, strlen(utf8str1),
+								   utf8str2, strlen(utf8str2));
+
+		pfree(utf8str1);
+		pfree(utf8str2);
+
+		/*
+		 * When all Unicode codepoints are equal, return result of binary
+		 * comparison.  In some edge cases, same characters may have different
+		 * representations in encoding.  Then our behavior could diverge from
+		 * standard.  However, that allow us to do simple binary comparison
+		 * for "==" operator, which is performance critical in typical cases.
+		 * In future to implement strict standard conformance, we can do
+		 * normalization of input JSON strings.
+		 */
+		if (cmp == 0)
+			return binaryCompareStrings(mbstr1, mblen1, mbstr2, mblen2);
+		else
+			return cmp;
+	}
+}
+
 /*
  * Compare two SQL/JSON items using comparison operation 'op'.
  */
@@ -2022,9 +2089,8 @@ compareItems(int32 op, JsonbValue *jb1, JsonbValue *jb2)
 						   jb2->val.string.val,
 						   jb1->val.string.len) ? jpbFalse : jpbTrue;
 
-			cmp = varstr_cmp(jb1->val.string.val, jb1->val.string.len,
-							 jb2->val.string.val, jb2->val.string.len,
-							 DEFAULT_COLLATION_OID);
+			cmp = compareStrings(jb1->val.string.val, jb1->val.string.len,
+								 jb2->val.string.val, jb2->val.string.len);
 			break;
 
 		case jbvBinary:

src/test/regress/sql/jsonb_jsonpath.sql

@@ -387,3 +387,19 @@ SELECT jsonb_path_match('[true, true]', '$[*]', silent => false);
 SELECT jsonb '[{"a": 1}, {"a": 2}]' @@ '$[*].a > 1';
 SELECT jsonb '[{"a": 1}, {"a": 2}]' @@ '$[*].a > 2';
 SELECT jsonb_path_match('[{"a": 1}, {"a": 2}]', '$[*].a > 1');
+
+-- test string comparison (Unicode codepoint collation)
+WITH str(j, num) AS
+(
+	SELECT jsonb_build_object('s', s), num
+	FROM unnest('{"", "a", "ab", "abc", "abcd", "b", "A", "AB", "ABC", "ABc", "ABcD", "B"}'::text[]) WITH ORDINALITY AS a(s, num)
+)
+SELECT
+	s1.j, s2.j,
+	jsonb_path_query_first(s1.j, '$.s < $s', vars => s2.j) lt,
+	jsonb_path_query_first(s1.j, '$.s <= $s', vars => s2.j) le,
+	jsonb_path_query_first(s1.j, '$.s == $s', vars => s2.j) eq,
+	jsonb_path_query_first(s1.j, '$.s >= $s', vars => s2.j) ge,
+	jsonb_path_query_first(s1.j, '$.s > $s', vars => s2.j) gt
+FROM str s1, str s2
+ORDER BY s1.num, s2.num;