Speed up uuid_out() by not relying on a StringInfo

Since the size of the string representation of an uuid is fixed, there
is no benefit in using a StringInfo.  This commit simplifies uuid_oud()
to not rely on a StringInfo, where avoiding the overhead of the string
manipulation makes the function substantially faster.

A COPY TO on a relation with one UUID attribute can show up to a 40%
speedup when the bottleneck is the COPY computation with uuid_out()
showing up at the top of the profiles (numbered measure here, Laurenz
has mentioned something closer to 20% faster runtimes), for example when
the data is fully in shared buffers or the OS cache.

Author: Laurenz Albe
Reviewed-by: Andres Freund, Michael Paquier
Description: https://postgr.es/m/679d5455cbbb0af667ccb753da51a475bae1eaed.camel@cybertec.at

src/backend/utils/adt/uuid.c

@@ -53,10 +53,13 @@ uuid_out(PG_FUNCTION_ARGS)
 {
 	pg_uuid_t  *uuid = PG_GETARG_UUID_P(0);
 	static const char hex_chars[] = "0123456789abcdef";
-	StringInfoData buf;
+	char	   *buf,
+			   *p;
 	int			i;
 
-	initStringInfo(&buf);
+	/* counts for the four hyphens and the zero-terminator */
+	buf = palloc(2 * UUID_LEN + 5);
+	p = buf;
 	for (i = 0; i < UUID_LEN; i++)
 	{
 		int			hi;
@@ -68,16 +71,17 @@ uuid_out(PG_FUNCTION_ARGS)
 		 * ("-"). Therefore, add the hyphens at the appropriate places here.
 		 */
 		if (i == 4 || i == 6 || i == 8 || i == 10)
-			appendStringInfoChar(&buf, '-');
+			*p++ = '-';
 
 		hi = uuid->data[i] >> 4;
 		lo = uuid->data[i] & 0x0F;
 
-		appendStringInfoChar(&buf, hex_chars[hi]);
-		appendStringInfoChar(&buf, hex_chars[lo]);
+		*p++ = hex_chars[hi];
+		*p++ = hex_chars[lo];
 	}
+	*p = '\0';
 
-	PG_RETURN_CSTRING(buf.data);
+	PG_RETURN_CSTRING(buf);
 }
 
 /*