From 5035701e07e8bd395aa878465a102afd7b74e8c3 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Sat, 26 Apr 2014 15:11:10 -0400
Subject: [PATCH] Improve generation algorithm for database system identifier.
As noted some time ago, the original coding had a typo ("|" for "^")
that made the result less unique than intended. Even the intended
behavior is obsolete since it was based on wanting to produce a
usable value even if we didn't have int64 arithmetic --- a limitation
we stopped supporting years ago. Instead, let's redefine the system
identifier as tv_sec in the upper 32 bits (same as before), tv_usec
in the next 20 bits, and the low 12 bits of getpid() in the remaining
bits. This is still hardly guaranteed-universally-unique, but it's
noticeably better than before. Per my proposal at
<29019.1374535940@sss.pgh.pa.us>
---
src/backend/access/transam/xlog.c | 13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 88ad51f9e7..a636bb6d2b 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -4889,15 +4889,16 @@ BootStrapXLOG(void)
* field, as being about as unique as we can easily get. (Think not to
* use random(), since it hasn't been seeded and there's no portable way
* to seed it other than the system clock value...) The upper half of the
- * uint64 value is just the tv_sec part, while the lower half is the XOR
- * of tv_sec and tv_usec. This is to ensure that we don't lose uniqueness
- * unnecessarily if "uint64" is really only 32 bits wide. A person
- * knowing this encoding can determine the initialization time of the
- * installation, which could perhaps be useful sometimes.
+ * uint64 value is just the tv_sec part, while the lower half contains the
+ * tv_usec part (which must fit in 20 bits), plus 12 bits from our current
+ * PID for a little extra uniqueness. A person knowing this encoding can
+ * determine the initialization time of the installation, which could
+ * perhaps be useful sometimes.
*/
gettimeofday(&tv, NULL);
sysidentifier = ((uint64) tv.tv_sec) << 32;
- sysidentifier |= (uint32) (tv.tv_sec | tv.tv_usec);
+ sysidentifier |= ((uint64) tv.tv_usec) << 12;
+ sysidentifier |= getpid() & 0xFFF;
/* First timeline ID is always 1 */
ThisTimeLineID = 1;
--
2.49.0