So essentially I think supporting special values like infinity boils down to trading away some small amount of performance -- more likely to be noticeable with JIT -- for some amount of possible programmer convenience. Some people may think that's a good trade, and other people may not like it. It just depends on whether the 'infinity' value is useful to you. If it is, you'll probably like the trade, because aside from the notational cost which Isaac mentioned, it's probably vastly faster to handle the infinity values in C code than to stick CASE..WHEN in to an SQL query. If it's not, you may dislike it. If your application code now has to know about the possibility 'infinity' value that it otherwise wouldn't have to worry about, you may dislike it for that reason also.
I'm not sure there's one right answer here - my personal feeling is that infinite values are a wart, but I grant Isaac's point that they can sometimes simplify SQL coding.
I have no objection to the introduction of another datatype that is stripped down for performance. I understand and agree with that need.
But the current datatypes do handle much complexity already. Blocking this proposal would not change that, IMHO. All that is being proposed is a small change to rationalize the existing code.
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Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services