Re: Next step towards 64bit XIDs: Switch to FullTransactionId for PGPROC->xid and XLogRecord->xl_xid

On Fri, Dec 29, 2023 at 02:36:19PM +0100, Matthias van de Meent wrote:
> I don't think this is an actionable change, as this wastes 4 more bytes (or
> 8 with alignment) in nearly all WAL records that don't use the
> HEAP/HEAP2/XLOG rmgrs, which would then be up to 10 (if not 14, when
> 64but-aligned) bytes per record. Unless something like [0] gets committed
> this will add a significant write overhead to all operations, even if they
> are not doing anything that needs an XID.

I was eyeing at the patches before reading your comment, and saw this
across the two patches:

@@ -176,7 +176,7 @@ struct PGPROC
Latch procLatch; /* generic latch for process */


- TransactionId xid; /* id of top-level transaction currently being
+ FullTransactionId xid; /* id of top-level transaction currently being
[...]
typedef struct XLogRecord
{
uint32 xl_tot_len; /* total len of entire record */
- TransactionId xl_xid; /* xact id */
+ pg_crc32c xl_crc; /* CRC for this record */
+ FullTransactionId xl_xid; /* xact id */

And FWIW, echoing with Matthias, making these generic structures
arbitrary larger is a non-starter. We should try to make them
shorter.
--
Michael