IMPALA-9531 dropped support for "dateless timestamps", e.g. cast("12:05:05" as timestamp) now returns NULL.
This led to breaking functions like minute("12:05:05"), as minute() expects a timestamp, and Impala adds an implicit cast, so what actually happens is minute(cast("12:05:05" as timestamp)), which returns NULL. The same expression works in Hive and other databases like mySQL.
IMO the best solution would be to add overloads for similar functions that take STRING as argument:
- this would solve the issue above
- would be consistent with Hive where minute() expects a STRING, not a TIMESTAMP
- it would be also faster, as the string->timestamp parsing is done anyway during casting, and extracting the minute part of a timestamp needs an a large integer division
At the first glance this should be done for the following functions:
TRUNC(TIMESTAMP / DATE ts, STRING unit) would also make sense, though I am not sure if it ever worked with dateless timestamp (in Hive this also takes STRING)
For functions like MONTH this is not needed for correctness, as we can convert "timeless dates" to timestamps or dates without problem. For performance it would still make sense to work on STRINGs directly.