stay Oracle in , What is a compressed index ？
Oracle Databases can use key compression （Key Compression） To compress B-Tree The part of an index or index that organizes the values of a primary key column in a table . Key compression can greatly reduce the space used by the index , An index that uses key compression is called a compressed index . Compression of the index is more about saving storage space , Reduce I/O Time . Compression also introduces storage overhead , It's just that a lot of the time compression saves more space than compression requires , So the overall storage cost is reduced after compression .
You can use the following SQL Rebuild the index into a compressed or uncompressed index ：
1ALTER INDEX EMPLOYEE_LAST_NAME_IDX REBUILD NOCOMPRESS;-- Uncompressed 2ALTER INDEX EMPLOYEE_LAST_NAME_IDX REBUILD COMPRESS;-- Compress
All compressed indexes can be accessed through the following SQL Statement get ：
1SELECT * FROM DBA_INDEXES D WHERE D.COMPRESSION='ENABLED';