Suppose T is a partitioned table on ds, where ds is a string column, the following queries:
SELECT a.* FROM T a WHERE a.ds=2008-09-08 LIMIT 1;
SELECT a.* FROM T a WHERE a.ds=2008-11-10 LIMIT 1;
return the first row from the first partition.
This is because of the typecast to double.
for a.ds=2008-01-01 or anything (a.ds=1),
evaluate (Double, Double) is invoked at partition pruning.
Since '2008-11-01' is not a valid double, it is converted to a null, and therefore the result of pruning returns null (unknown) - not FALSE.
All unknowns are also accepted, therefore all partitions are accepted which explains this behavior.
filter is not invoked since it is a select * query, so map-reduce job is started.
We just turn off this optimization if pruning indicates that there can be unknown partitions.