Change xl_hash_vacuum_one_page.ntuples from int to uint16

[Drouvot, Bertrand]

Hi hackers,

While working on [1], I noticed that xl_hash_vacuum_one_page.ntuples is an int.

Unless I'm missing something, It seems to me that it would make more sense to 
use an uint16 (like this is done for
gistxlogDelete.ntodelete for example).

Please find attached a patch proposal to do so.

While that does not currently change the struct size:

No patch:

(gdb) ptype /o struct xl_hash_vacuum_one_page
/* offset      |    size */  type = struct xl_hash_vacuum_one_page {
/*      0      |       4 */    TransactionId snapshotConflictHorizon;
/*      4      |       4 */    int ntuples;

                               /* total size (bytes):    8 */
                             }
With patch:

(gdb) ptype /o struct xl_hash_vacuum_one_page
/* offset      |    size */  type = struct xl_hash_vacuum_one_page {
/*      0      |       4 */    TransactionId snapshotConflictHorizon;
/*      4      |       2 */    uint16 ntuples;
/* XXX  2-byte padding   */

                               /* total size (bytes):    8 */
                             }

It could reduce it when adding new fields (like this is is done in [1]).

We would get:

No patch:

(gdb) ptype /o struct xl_hash_vacuum_one_page
/* offset      |    size */  type = struct xl_hash_vacuum_one_page {
/*      0      |       4 */    TransactionId snapshotConflictHorizon;
/*      4      |       4 */    int ntuples;
/*      8      |       1 */    _Bool isCatalogRel;
/* XXX  3-byte padding   */

                               /* total size (bytes):   12 */
                             }

With patch:

(gdb) ptype /o struct xl_hash_vacuum_one_page
/* offset      |    size */  type = struct xl_hash_vacuum_one_page {
/*      0      |       4 */    TransactionId snapshotConflictHorizon;
/*      4      |       2 */    uint16 ntuples;
/*      6      |       1 */    _Bool isCatalogRel;
/* XXX  1-byte padding   */

                               /* total size (bytes):    8 */
                             }

Means saving 4 bytes in that case.

Looking forward to your feedback,

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com
diff --git a/src/include/access/hash_xlog.h b/src/include/access/hash_xlog.h
index a2f0f39213..9894ab9afe 100644
--- a/src/include/access/hash_xlog.h
+++ b/src/include/access/hash_xlog.h
@@ -251,13 +251,13 @@ typedef struct xl_hash_init_bitmap_page
 typedef struct xl_hash_vacuum_one_page
 {
        TransactionId snapshotConflictHorizon;
-       int                     ntuples;
+       uint16                  ntuples;
 
        /* TARGET OFFSET NUMBERS FOLLOW AT THE END */
 } xl_hash_vacuum_one_page;
 
 #define SizeOfHashVacuumOnePage \
-       (offsetof(xl_hash_vacuum_one_page, ntuples) + sizeof(int))
+       (offsetof(xl_hash_vacuum_one_page, ntuples) + sizeof(uint16))
 
 extern void hash_redo(XLogReaderState *record);
 extern void hash_desc(StringInfo buf, XLogReaderState *record);