Use of some built-in methods in oracle

Use of some built-in methods in oracle 1. query a tablespace in a database: Use select * from DBA_INDEXES or DBA_OBJECTS 2. query database connections: select count (*) from V $ SESSION 3. query the maximum number of connections in the database: select * from V $ PARAMETER find name = 'processs' in the query list. 4. query the table information in the currently logged-on database: select * from USER_IND_COLUMNS or USER_INDEXES 5. query the field name information of a table: select * from user_tab_columns where table_name = 'query table name'