On 2010-10-26, John R Pierce <pierce(at)hogranch(dot)com> wrote:
>
> count(*) has to read the whole table to get the accurate count. The
> reason for this is that different clients can see different versions of
> that table, for instance, if client A is already in a transaction, and
> client B then does an INSERT, the two clients will see different values
> for the count.
They may or may not. the default transaction isolation level "read commited"
allows a session to see most changes that were committed externally after the
start of the transaction.
Tlso the index may include deleted rows.
which is another reason count(*) does a table scan.
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