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Previously, convert_one_string_to_scalar() would examine up to 20 bytes of the input string, producing a scalar conversion with theoretical precision far greater than is of any possible use considering the other limitations on the accuracy of the resulting selectivity estimate. (I think this choice might pre-date the caller-level logic that strips any common prefix of the strings; before that, there could have been value in scanning the strings far enough to use all the precision available in a double.) Aside from wasting cycles to little purpose, this choice meant that the "denom" variable could grow to as much as 256^21 = 3.74e50, which could overflow in some non-IEEE float arithmetics. While we don't really support any machines with non-IEEE arithmetic anymore, this still seems like quite an unnecessary platform dependency. Limit the scan to 12 bytes instead, thus limiting "denom" to 256^13 = 2.03e31, a value more likely to be computable everywhere. Per testing by Greg Stark, which showed overflow failures in our standard regression tests on VAX. |
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