| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | "Dann Corbit" <DCorbit(at)connx(dot)com> |
| Cc: | "Doug McNaught" <doug(at)wireboard(dot)com>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: Suggestion for optimization |
| Date: | 2002-04-05 19:51:41 |
| Message-ID: | 9605.1018036301@sss.pgh.pa.us |
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"Dann Corbit" <DCorbit(at)connx(dot)com> writes:
> How many accounts are 90 days overdue? Bill says 78 and Frank says 82.
> Who is right and how can we know?
If Bill and Frank look at exactly the same instant (ie, from read-only
transactions started at the same time), they will get the same answer.
If they are looking from transactions started at different times, or
from transactions that have themselves added/removed rows, they won't
necessarily get the same answer. That does not make either answer
wrong, just a reflection of the snapshots they are seeing.
regards, tom lane
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