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Commit5035701

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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 Even the intendedbehavior is obsolete since it was based on wanting to produce ausable value even if we didn't have int64 arithmetic --- a limitationwe stopped supporting years ago. Instead, let's redefine the systemidentifier as tv_sec in the upper 32 bits (same as before), tv_usecin the next 20 bits, and the low 12 bits of getpid() in the remainingbits. This is still hardly guaranteed-universally-unique, but it'snoticeably better than before. Per my proposal at<29019.1374535940@sss.pgh.pa.us>
1 parent528c454 commit5035701

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  • src/backend/access/transam

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‎src/backend/access/transam/xlog.c

Lines changed: 7 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -4889,15 +4889,16 @@ BootStrapXLOG(void)
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* field, as being about as unique as we can easily get. (Think not to
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* use random(), since it hasn't been seeded and there's no portable way
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* to seed it other than the system clock value...) The upper half of the
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* uint64 value is just the tv_sec part, while the lower halfis the XOR
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*of tv_sec and tv_usec. This is to ensure that we don't lose uniqueness
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*unnecessarily if "uint64" is really only 32 bits wide. A person
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*knowing this encoding candetermine the initialization time of the
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*installation, which couldperhaps be useful sometimes.
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* uint64 value is just the tv_sec part, while the lower halfcontains the
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*tv_usec part (which must fit in 20 bits), plus 12 bits from our current
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*PID for a little extra uniqueness. A person knowing this encoding can
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* determine the initialization time of the installation, which could
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* perhaps be useful sometimes.
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*/
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gettimeofday(&tv,NULL);
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sysidentifier= ((uint64)tv.tv_sec) <<32;
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sysidentifier |= (uint32) (tv.tv_sec |tv.tv_usec);
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sysidentifier |= ((uint64)tv.tv_usec) <<12;
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sysidentifier |=getpid()&0xFFF;
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/* First timeline ID is always 1 */
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ThisTimeLineID=1;

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