pgsql: Doc: fix ancient mistake, or at least obsolete info, in rules ex

Doc: fix ancient mistake, or at least obsolete info, in rules example.

The example of expansion of multiple views claimed that the resulting
subquery nest would not get fully flattened because of an aggregate
function. There's no aggregate in the example, though, only a user
defined function confusingly named MIN(). In a modern server, the
reason for the non-flattening is that MIN() is volatile, but I'm
unsure whether that was true back when this text was written.

Let's reduce the confusion level by using LEAST() instead (which
we didn't have at the time this example was created). And then
we can just say that the planner will flatten the sub-queries, so
the rewrite system doesn't have to.

Noted by Paul Jungwirth. This text is old enough to vote, so
back-patch to all supported branches.

Discussion: https://postgr.es/m/CA+renyXZFnmp9PcvX1EVR2dR=XG5e6E-AELr8AHCNZ8RYrpnPw@mail.gmail.com

Branch
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REL9_4_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/84aad60b9cdade0a9d55f938bec92d788e2c54e9

Modified Files
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doc/src/sgml/rules.sgml | 32 ++++++++------------------------
1 file changed, 8 insertions(+), 24 deletions(-)