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Partitioning enhancements
No database release is complete without partitioning enhancements, and Database 12c is not an exception.
Traditionally, indexes are created in all partitions of a partitioned table. Version 12c introduces Partial Indexing, which allows you to create indexes on a partial set of partitions. For example, if you have partitions p1, p2, p3 ... p10 in a table, you could choose to create an index on partitions p1, p2, and p3 only, and not to create indexes on the other partitions. This feature will be useful in time-variant partitioning schemes, typically in which older partitions are mostly only queried and newer partitions are updated heavily.
We can reduce transaction workload by deferring indexes on transaction-intensive partitions and add indexes only when the partitions become less transaction-intensive. Also, the user can build new indexes in multiple steps to reduce locking and resource consumption issues associated with a traditional index rebuild. Of course, you probably would have guessed, partition pruning is a key requirement for the Optimizer to choose partial indexes, and partial indexes are not supported for unique indexes.
Global indexes cause problems for partition-level operations. For example, if a partition is dropped, then a global index on that table must be updated. While the drop/truncate partition commands are fast (because they are DDL operations), updates to global index entries are far slower, leading to availability issues during DDL operations. For this reason, Version 12c decouples global index maintenance from DDL command execution. Thus, only global index metadata is updated during partition DDL operation, and actual index maintenance can be deferred for a later time. A new scheduled job lets you clear stale global index entries at, say, 2 a.m., improving the viability of global indexes on critical tables.
Also in Database 12c, reference partitioning now truncates dependent partitions. With a reference partitioning scheme, tables in both ends of a referential constraint are partitioned along the same partitioning boundaries. From 12c onward, truncating or dropping a parent table partition will cascade to child table partitions.
Backup and recovery
Traditionally, restoring a single table is a cumbersome process involving a tablespace restore, exporting the restored table
from the restored tablespace, and then importing to the production database. The new restore table command in Recovery Manager
(RMAN) simplifies this task.