alter index rebuild与alter index rebuild online的区别

alter index rebuild online 实质上是扫描表而不是扫描现有的索引块来实现索引的重建；在重建的过程中，允许对表做 DML 操作；花费的时间较长；

alter index rebuild 只扫描现有的索引块来实现索引的重建；在重建的过程中，不允许对表有任何DML操作；花费时间较短。

可以用10046事件来发现这个现象。

metalink Note:272762.1

Problem:
========
- Online Index rebuild takes a long time.
- ONLINE INDEX REBUILD SCANS THE BASE TABLE AND NOT THE INDEX

Symptoms:
=========
Performance issues while rebuilding very large indexes.
- The offline rebuilds of their index is relatively quick -finishes in 15 minutes.
- Issuing index rebuild ONLINE statement => finishes in about an hour.
- This behavior of ONLINE index rebuilds makes it a non-option for large tables
as it just takes too long to scan the table to rebuild the index. The
offline may not be feasible due to due to the 24/7 nature of the database.
- This may be a loss of functionality for such situations.
- If we attempt to simultaneously ONLINE rebuild the same indexes we may encounter
hanging behavior indefinitely (or more than 6 hours).

DIAGNOSTIC ANALYSIS:
--------------------
We can trace the sessions rebuilding the indexes with 10046 level 12.
Comparing the IO reads for the index-rebuild and the index-rebuild-online
reveals the following:

-ONLINE index rebuilds
It scans the base table and it doesn't scan the blocks of the index.

-OFFLINE index rebuilds
It scans the index for the build operation.

- This behaviour is across all versions.

Cause
Cause/Explanation
=============
When you rebuild index online,
- it will do a full tablescan on the base table.
- At the same time it will maintain a journal table for DML data, which has
changed during this index rebuilding operation.
So it should take longer time, specially if you do lots of DML on the same table,
while rebuilding index online.

On the other hand, while rebuilding the index without online option, Oracle will grab
the index in X-mode and rebuild a new index segment by selecting the data from
the old index. So here we are
- not allowing any DML on the table hence there is no journal table involved
- and it is doing an index scan
Hence it will be pretty fast.
Fix
Solution/Conclusion:
===========
- The ONLINE index rebuild reads the base table, and this is by design.
- Rebuilding index ONLINE is pretty slow.
- Rebuilding index offline is very fast, but it prevents any DML on the base table.