Teach convert() and friends to avoid copying when possible.

Presently, pg_convert() allocates a new bytea and copies the result
regardless of whether any conversion actually happened.  This
commit adjusts this function to return the source pointer as-is if
no conversion occurred.  This optimization isn't expected to make a
tremendous difference, but it still seems worthwhile to avoid
unnecessary memory allocations.

Author: Yurii Rashkovskii
Reviewed-by: Bertrand Drouvot
Discussion: https://postgr.es/m/CA%2BRLCQyknBPSWXRBQGOi6aYEcdQ9RpH9Kch4GjoeY8dQ3D%2Bvhw%40mail.gmail.com

src/backend/utils/mb/mbutils.c

@@ -585,19 +585,19 @@ pg_convert(PG_FUNCTION_ARGS)
 												  src_encoding,
 												  dest_encoding);
 
-	/* update len if conversion actually happened */
-	if (dest_str != src_str)
-		len = strlen(dest_str);
+
+	/* return source string if no conversion happened */
+	if (dest_str == src_str)
+		PG_RETURN_BYTEA_P(string);
 
 	/*
 	 * build bytea data type structure.
 	 */
+	len = strlen(dest_str);
 	retval = (bytea *) palloc(len + VARHDRSZ);
 	SET_VARSIZE(retval, len + VARHDRSZ);
 	memcpy(VARDATA(retval), dest_str, len);
-
-	if (dest_str != src_str)
-		pfree(dest_str);
+	pfree(dest_str);
 
 	/* free memory if allocated by the toaster */
 	PG_FREE_IF_COPY(string, 0);