Revert "Use text_substr for getting top comment"

This reverts commit 9531cfd3bf.

src/backend/distributed/utils/attribute.c

@@ -63,6 +63,7 @@ static int CreateTenantStats(MultiTenantMonitor *monitor);
 static int FindTenantStats(MultiTenantMonitor *monitor);
 static size_t MultiTenantMonitorshmemSize(void);
 static char * ExtractTopComment(const char *inputString);
+static char * Substring(const char *str, int start, int end);
 static char * EscapeCommentChars(const char *str);
 static char * UnescapeCommentChars(const char *str);
 
@@ -645,14 +646,7 @@ ExtractTopComment(const char *inputString)
 
 	if (commentEndCharsIndex > commentStartCharsLength)
 	{
-		Datum substringTextDatum = DirectFunctionCall3(text_substr, PointerGetDatum(
-														   cstring_to_text(inputString)),
-													   Int32GetDatum(
-														   commentStartCharsLength + 1),
-													   Int32GetDatum(
-														   commentEndCharsIndex -
-														   commentStartCharsLength));
-		return TextDatumGetCString(substringTextDatum);
+		return Substring(inputString, commentStartCharsLength, commentEndCharsIndex);
 	}
 	else
 	{
@@ -661,6 +655,31 @@ ExtractTopComment(const char *inputString)
 }
 
 
+/* Extracts a substring from the input string between the specified start and end indices.*/
+static char *
+Substring(const char *str, int start, int end)
+{
+	int len = strlen(str);
+
+	/* Ensure start and end are within the bounds of the string */
+	if (start < 0 || end > len || start > end)
+	{
+		return NULL;
+	}
+
+	/* Allocate memory for the substring */
+	char *substr = (char *) palloc((end - start + 1) * sizeof(char));
+
+	/* Copy the substring to the new memory location */
+	strncpy_s(substr, end - start + 1, str + start, end - start);
+
+	/* Add null terminator to end the substring */
+	substr[end - start] = '\0';
+
+	return substr;
+}
+
+
 /*  EscapeCommentChars adds a backslash before each occurrence of '*' or '/' in the input string */
 static char *
 EscapeCommentChars(const char *str)