The Database Explorer window has two filters:
Information about how to specify filters can be seen at Filters.
You can use the Schema Filter to reduce the number schemas shown in the schema-list for an object.
For the Schema Filter, you can specify multiple filters, separated by a comma. For instance:
this will show all schemas containing CAT plus all those starting with TEST.
You can specify that AQT remembers your schema filters from one AQT session to the next. This is done with Options > Database Explorer > Remember schema filters between sessions
The schema filter operates quite differently from the Object filter, and other filters in AQT. With these, items are shown in a grid, and the grid rows are made visible / not visible according to the filter.
Schemas as shown in a Tree, and with this, items cannot made not-visible. As a result, the schema-list will be rebuilt every time the filter changes. This can be a time-consuming operation if you have a large number of schemas. A better option is to double-click the filter box then specify the filter on the filter dialog.
Secondly, filtering will run a lot faster if you are using Caching of the schemas. If caching is not being used, AQT will need to fetch the schemas from the database whenever the filter is changed. This is also a time-consuming operation.
As a final point, AQT will retrieve all schemas from the database and apply the filter to determine which of these is displayed. If you wish AQT to only retrieve a limited number of schemas from the database then this can be done by amending the queries in the config file for the database.