My Query is running Slow , but it has stats up to date . Can we expect fragmentation issue

 Yes, a table can still become fragmented even if the statistics are up-to-date. Fragmentation can occur when data is inserted, updated or deleted from a table. When data is modified, it may not fit into the same space as the original data, leading to fragmentation of the table.

To fix table fragmentation issues in Oracle, you can use one or more of the following methods:

Fragmentation is a common issue in Oracle databases that occurs when a table or index has allocated space that is not contiguous or is not efficiently used. Fragmentation can lead to performance issues, such as slower query response times and increased disk I/O. There are several types of fragmentation that can occur in an Oracle database:

Extent fragmentation: This occurs when a table or index has free space that is not contiguous. This can happen when rows are deleted from the table, or when the table is resized. Extent fragmentation can cause performance issues because the database has to search multiple extents to retrieve data.

Block fragmentation: This occurs when a table or index has free space within its data blocks. Block fragmentation can occur when rows are updated or deleted, causing empty space within a data block. Block fragmentation can cause performance issues because the database has to search multiple data blocks to retrieve data.

Space fragmentation: This occurs when a table or index has free space that is not efficiently used. For example, if a table has a large number of small extents, the database may have to perform more I/O operations to retrieve data. Space fragmentation can also occur when the table has a high degree of parallelism, which can lead to suboptimal space allocation.

To fix table fragmentation, you can rebuild the table or its indexes. The rebuild operation creates a new table or index and moves the data to it, thus removing any fragmentation that might have occurred.

Here are the steps to check for table fragmentation and fix it:

Check table fragmentation:

SELECT t.owner, t.table_name, t.blocks, f.minsize, f.maxsize, f.extents

FROM dba_tables t, dba_free_space f

WHERE t.table_name = 'your_table_name'

AND t.owner = 'your_table_owner'

AND f.tablespace_name = t.tablespace_name

AND f.block_id <= t.blocks

AND f.block_id + f.blocks > t.blocks;

If the query returns a large number of extents or a low value for the maximum free space, it indicates that the table is fragmented and may require defragmentation.

Note that this query only checks for fragmentation at the table level. To check for fragmentation at the index level, you can use a similar query with the DBA_INDEXES and DBA_FREE_SPACE views.

Here are the different ways to defragment a table in an Oracle database:

Rebuilding the Table:

The first method is to rebuild the table, which involves creating a new table and moving the data to it, thus eliminating any fragmentation. This method can be resource-intensive and may impact performance during the rebuild operation. You can use the ALTER TABLE statement with the MOVE option to rebuild a table.

ALTER TABLE employees MOVE TABLESPACE new_tablespace;

Rebuilding the Indexes:

The second method is to rebuild the indexes, which involves dropping and recreating the indexes. This method is less resource-intensive than rebuilding the entire table and can be performed online while users are still accessing the table. You can use the ALTER INDEX statement with the REBUILD option to rebuild an index.

ALTER INDEX emp_name_idx REBUILD;

Coalescing Free Space:

The third method is to coalesce the free space in the table, which involves moving the data to eliminate gaps between rows and pages. This method can be useful for tables with a high degree of updates and deletes. You can use the ALTER TABLE statement with the DEALLOCATE UNUSED option to coalesce free space in a table.

ALTER TABLE employees DEALLOCATE UNUSED;

Rebuild the table: You can use the ALTER TABLE command with the MOVE option to rebuild the table and consolidate its free space. For example:

ALTER TABLE your_table_name MOVE TABLESPACE your_tablespace_name;

This command moves the table to a new location in the tablespace and reclaims any free space that may be fragmented.

Use the Segment Advisor: Oracle provides a Segment Advisor tool that analyzes tables and indexes for fragmentation and provides recommendations for reducing fragmentation. You can use this tool to identify fragmented objects and take corrective action. For example:

BEGIN

  DBMS_SPACE.ADVISE_SEGMENT(segment_owner => 'your_table_owner', segment_name => 'your_table_name', advice_list => DBMS_SPACE.ADVISE_REBUILD);

END;

This command invokes the Segment Advisor for the specified table and provides advice for rebuilding the table to reduce fragmentation.

Use the Online Table Redefinition feature: This feature allows you to rebuild a table online, while the table remains accessible to users. You can use the DBMS_REDEFINITION package to redefine the table, and Oracle automatically manages the migration of data from the old to the new table. For example:

BEGIN

  DBMS_REDEFINITION.START_REDEF_TABLE(tab_name => 'your_table_name', int_table_flags => DBMS_REDEFINITION.CONS_USE_ROWID);

END;

This command starts the online table redefinition process for the specified table.

Use the Export-Import method: You can export the table to a dump file, drop the original table, and then import the table back into the database. This method can be useful if you need to perform other maintenance tasks on the table, such as changing the tablespace or resizing the datafiles. For example:

expdp your_user/password@your_database tables=your_table_name directory=your_export_directory dumpfile=your_export_file.dmp

DROP TABLE your_table_name;

impdp your_user/password@your_database directory=your_import_directory dumpfile=your_export_file.dmp

These commands export the table, drop the original table, and then import the table back into the database.