Derby optionally creates database tables to hold statistics information captured using XPLAIN style. You can have zero, one, or many sets of these tables; each set of tables is stored in a separate schema. The schema which is used for capturing statement execution information is specified using the SYSCS_UTIL.SYSCS_SET_XPLAIN_SCHEMA system procedure
You can query these tables to analyze the behavior of statement execution.
All of the above system tables reside in the schema which you specified. Because this is not the default schema, qualify all queries accessing the system tables with the schema name.
You can create the schema and tables ahead of time if you wish, but usually it is easier to let Derby automatically create the schema and the tables for you. You can capture multiple sets of data into the same tables, or you can specify a different schema each time.
Tuning Java DB contains general information about how to use XPLAIN style and the XPLAIN database tables.