On 3/17/16 7:40 PM, Tom Lane wrote:
Chapman Flack <c...@anastigmatix.net> writes:It seems that a typmod can only be used restrict the set of possible values of the unmodified type (as clearly seen in the language "length conversion cast", since certainly a typmod allowing { string | length < N } is doing nothing but enforcing a subset of { string }. Each element of the subset is still a valid element of the whole set (naturally, boring) *and has to be represented the same way* (interesting): the representation mustn't do clever things that you would need to know the typmod in order to interpret, because most uses of a value are without access to the typmod.You do need to be able to interpret values of the type without having separate access to the typmod, but I don't think it follows that it's as restrictive as you say. One easy way around that is to store the typmod in the value. Practical uses might include compressing the data in different ways depending on typmod. I'm drawing a blank on other compelling examples though I'm sure there are some. Have you looked at PostGIS? I'm pretty sure some of their types make use of typmod in nontrivial ways.
If you want a non-trivial use of typmod, take a look at the (work in progress) variant type I created[1]. It allows you pass names of "registered variants" in via typmod. The idea behind that is to restrict what types you can actually store in a particular variant field (though you can also disallow a registered variant from being used in a table definition).
I did run into some cases where Postgres ignored typmod, so I special case the default typmod (-1) to a registered variant that's disabled.
[1] https://github.com/BlueTreble/variant/blob/master/doc/variant.md#variant-modifier
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