Casting any value obtained from an Osisoft PI datasource results in NULL, regardless of the original value.
E.g. a query such as

select cast(stringnum as bigdecimal), cast(intnum as bigdecimal), cast(shortvalue as bigdecimal) from Source.smalla

returns result set full of NULLs.

The SOURCE SRC COMMAND in teiid-command.log for this query is however

SELECT g_0.StringNum, g_0.IntNum, g_0.ShortValue FROM dvqe..SmallA AS g_0

i.e. the cast is not pushed down (as Osisoft doesn't support bigdecimal datatype).