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Commitaad663a

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Reduce number of bytes examined by convert_one_string_to_scalar().
Previously, convert_one_string_to_scalar() would examine up to 20 bytes ofthe input string, producing a scalar conversion with theoretical precisionfar greater than is of any possible use considering the other limitationson the accuracy of the resulting selectivity estimate. (I think thischoice might pre-date the caller-level logic that strips any common prefixof the strings; before that, there could have been value in scanning thestrings 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 couldoverflow in some non-IEEE float arithmetics. While we don't really supportany machines with non-IEEE arithmetic anymore, this still seems like quitean unnecessary platform dependency. Limit the scan to 12 bytes instead,thus limiting "denom" to 256^13 = 2.03e31, a value more likely to becomputable everywhere.Per testing by Greg Stark, which showed overflow failures in our standardregression tests on VAX.
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‎src/backend/utils/adt/selfuncs.c

Lines changed: 9 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -3861,10 +3861,16 @@ convert_one_string_to_scalar(char *value, int rangelo, int rangehi)
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return0.0;/* empty string has scalar value 0 */
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/*
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* Since base is at least 10, need not consider more than about 20 chars
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* There seems little point in considering more than a dozen bytes from
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* the string. Since base is at least 10, that will give us nominal
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* resolution of at least 12 decimal digits, which is surely far more
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* precision than this estimation technique has got anyway (especially in
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* non-C locales). Also, even with the maximum possible base of 256, this
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* ensures denom cannot grow larger than 256^13 = 2.03e31, which will not
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* overflow on any known machine.
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*/
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if (slen>20)
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slen=20;
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if (slen>12)
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slen=12;
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/* Convert initial characters to fraction */
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base=rangehi-rangelo+1;

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