Thread: pgsql: Document evaluation-order considerations for aggregate functions

Document evaluation-order considerations for aggregate functions.

The SELECT reference page didn't really address the question of when
aggregate function evaluation occurs, nor did the "expression evaluation
rules" documentation mention that CASE can't be used to control whether
an aggregate gets evaluated or not.  Improve that.

Per discussion of bug #11661.  Original text by Marti Raudsepp and Michael
Paquier, rewritten significantly by me.

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/df3e23e1b509001ac9336763404406bdcad04dcc

Modified Files
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doc/src/sgml/ref/select.sgml |   36 ++++++++++++++++++++++++++----------
doc/src/sgml/syntax.sgml     |   22 ++++++++++++++++++++++
2 files changed, 48 insertions(+), 10 deletions(-)