If there are multiple query operators manipulating the tables in the same database, should one use the single JDBC Table construct and have all query operators access it?

Or is it better to have one JDBC Table construct per Database table?

What are the performance considerations? Or it doesn't make any difference?

Using one vs many JDBC Table constructs

One JDBC connection is made per Query Operator for a given JDBC Table element unless you have set for that data-source:
  jdbc-share-connection = true
If you set this option, there is no difference whether you use one or many separate Query operators. You may still use separate JDBC Table elements for a given data-source in order to separate Query operators into separate JDBC connections to isolate very active queries from less-active but latency sensitive queries.

The above setting is described here:
TIBCO StreamBase 7:  Help > StreamBase Documentation > StreamBase References > StreamBase Server Configuration File XML Reference
TIBCO Streaming 10: Help > TIBCO Streaming > Configuration Guide > StreamBase Configuration Types > StreamBase JDBC DataSource Configuration