On 05/27/2014 10:12 PM, Evan Jones wrote:
I was reading the Postgres MVCC documentation today (which is
generally fantastic BTW), and am slightly confused by a single
sentence example, describing possible read-only snapshot isolation
anomalies. I would like to submit a patch to clarify this example,
since I suspect others may be also confused, but to do that I need
help understanding it. The example was added as part of the
Serializable Snapshot Isolation patch.
Link to the commit:
http://git.postgresql.org/gitweb/?p=postgresql.git;h=dafaa3efb75ce1aae2e6dbefaf6f3a889dea0d21
I'm referring to the following sentence of 13.2.2, which is still in
the source tree:
http://www.postgresql.org/docs/devel/static/transaction-iso.html#XACT-REPEATABLE-READ
"For example, even a read only transaction at this level may see a
control record updated to show that a batch has been completed but
not see one of the detail records which is logically part of the
batch because it read an earlier revision of the control record."
Hmm, that seems to be a super-summarized description of what Kevin & Dan
called the "receipts problem". There's an example of that in the
isolation test suite, see src/test/isolation/specs/receipt-report.spec.
Googling for it, I also found an academic paper written by Kevin & Dan
that illustrates it: http://arxiv.org/pdf/1208.4179.pdf, "2.1.2 Example
2: Batch Processing". (Nice work, I didn't know of that paper until now!)
I agree that's too terse. I think it would be good to actually spell out
a complete example of the Receipt problem in the manual. That chapter in
the manual contains examples of anomalities in Read Committed mode, so
it would be good to give a concrete example of an anomaly in Repeatable
Read mode too. Want to write up a docs patch?
- Heikki
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