TAILIEUCHUNG - Oracle Database Administration for Microsoft SQL Server DBAs part 21

Oracle Database Administration for Microsoft SQL Server DBAs part 21 takes the administration topics with which the SQL Server DBA is familiar, translates them into Oracle terms, and then expands on Oracle functionality. Definitions and comparative terms run throughout the book so the SQL Server DBA can easily leverage existing knowledge. This Oracle Press guide also expands on some of the features in Oracle that do not match up directly with SQL Server, and looks at other processes often performed on an Oracle database that would not typically be a standard practice in SQL Server environments | 182 Oracle Database Administration for Microsoft SQL Server DBAs In SQL Server clustered indexes are common and these help to reorganize fragmented tables. Rebuilding clustered indexes in SQL Server will place some locks and possibly some blocking on the index. Rebuilding the clustered index also reorganizes a table. The performance of online rebuilds has improved with newer versions of SQL Server. Oracle can use clustered indexes but it seems to be more common to use nonclustered indexes. Oracle has an Automatic Segment Space Management ASSM feature which has improved with each version and helps to reduce fragmentation during regular processing. As with SQL Server online rebuilds in newer versions of Oracle are more efficient. With SQL Server you can use DBCC commands to evaluate if an index should be rebuilt. DBCC SHOWCONTIG shows fragmentation for tables and a table with a clustered index probably has the same fragmentations. Also the system table dm_db_index_physical_stats can return average fragmentation for all of the indexes in the database. With Oracle the ANALYZE TABLE table_name VALIDATE STRUCTURE command makes sure the index is in sync with the table. When CASCADE is used with this command information will be inserted into an index_stats table which you can use to evaluate if indexes need to be rebuilt. f Sqlplus analyze table emp validate structure cascade Table analyzed. sqlplus select height blocks lf_rows del_lf_rows btree_space used_space from index_stats where name IDX_EMP1 HEIGHT BLOCKS LFROWS dellfrows BTREESPACE USEDSPACE 1 8 14 3 8000 209 The index_stats table shows the height of the index. As a general rule an index with a height great than 4 might be considered for a rebuild. Also look at the deleted leaf blocks del_fl_rows value. This amount should be under 20 percent of the total leaf rows. As noted earlier Oracle supplies advisors to help assess maintenance requirements. The Segment Advisor part of the default maintenance jobs reports on .

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