[postgis-devel] ST_Union specification questions

[Bryce L Nordgren]

So....

How are the p_, t_ and f_ expressions related? It seems like these
accumulate values in three different bands? If so, how was the number
"three bands" determined? If you don't specify one set of expressions, does
the corresponding band disappear out of the result? Or am I reading it
completely wrong: Does ST_Union always return a single band, and the
"state" expression is some sort of "scratch" band which is used to
accumulate information, but which is not actually returned?

Also, aggregate functions really should not produce results which depend on
the order in which the individual records are encountered. Allowing this
functionality (see specifications: FIRST and LAST set of expressions) is
inviting trouble of a spurious and random nature, particularly when results
from Windows are compared with results from linux. The answer should always
be the same, just like the geometry version of ST_Union always returns the
same value. The solution may be to limit the functions which are allowed to
appear in the expression to other aggregates...

Bryce
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