Analyzing Tables, Indexes, and Clusters

You analyze a schema object (table, index, or cluster) to:

• Collect and manage statistics for it
• Verify the validity of its storage format
• Identify migrated and chained rows of a table or cluster

Note:

Do not use the COMPUTE and ESTIMATE clauses of ANALYZE to collect optimizer statistics. These clauses are supported for backward compatibility. Instead, use the DBMS_STATS package, which lets you collect statistics in parallel, collect global statistics for partitioned objects, and fine tune your statistics collection in other ways. The cost-based optimizer, which depends upon statistics, will eventually use only statistics that have been collected by DBMS_STATS. See Oracle Database PL/SQL Packages and Types Reference for more information on the DBMS_STATS package. You must use the ANALYZE statement (rather than DBMS_STATS) for statistics collection not related to the cost-based optimizer, such as:

• To use the VALIDATE or LIST CHAINED ROWS clauses
• To collect information on freelist blocks

The following topics are discussed in this section:

• Using DBMS_STATS to Collect Table and Index Statistics
• Validating Tables, Indexes, Clusters, and Materialized Views
• Listing Chained Rows of Tables and Clusters

Using DBMS_STATS to Collect Table and Index Statistics

You can use the DBMS_STATS package or the ANALYZE statement to gather statistics about the physical storage characteristics of a table, index, or cluster. These statistics are stored in the data dictionary and can be used by the optimizer to choose the most efficient execution plan for SQL statements accessing analyzed objects.
Oracle recommends using the more versatile DBMS_STATS package for gathering optimizer statistics, but you must use the ANALYZE statement to collect statistics unrelated to the optimizer, such as empty blocks, average space, and so forth.
The DBMS_STATS package allows both the gathering of statistics, including utilizing parallel execution, and the external manipulation of statistics. Statistics can be stored in tables outside of the data dictionary, where they can be manipulated without affecting the optimizer. Statistics can be copied between databases or backup copies can be made.
The following DBMS_STATS procedures enable the gathering of optimizer statistics:

• GATHER_INDEX_STATS
  
• GATHER_TABLE_STATS
  
• GATHER_SCHEMA_STATS
  
• GATHER_DATABASE_STATS
  
  See Also:

  • Oracle Database Performance Tuning Guide for information about using DBMS_STATS to gather statistics for the optimizer
    
  • Oracle Database PL/SQL Packages and Types Reference for a description of the DBMS_STATS package
    

Validating Tables, Indexes, Clusters, and Materialized Views

To verify the integrity of the structure of a table, index, cluster, or materialized view, use the ANALYZE statement with the VALIDATE STRUCTURE option. If the structure is valid, no error is returned. However, if the structure is corrupt, you receive an error message.
For example, in rare cases such as hardware or other system failures, an index can become corrupted and not perform correctly. When validating the index, you can confirm that every entry in the index points to the correct row of the associated table. If the index is corrupt, you can drop and re-create it.
If a table, index, or cluster is corrupt, you should drop it and re-create it. If a materialized view is corrupt, perform a complete refresh and ensure that you have remedied the problem. If the problem is not corrected, drop and re-create the materialized view.
The following statement analyzes the emp table:

ANALYZE TABLE emp VALIDATE STRUCTURE;

You can validate an object and all dependent objects (for example, indexes) by including the CASCADE option. The following statement validates the emp table and all associated indexes:

ANALYZE TABLE emp VALIDATE STRUCTURE CASCADE;

By default the CASCADE option performs a complete validation. Because this operation can be resource intensive, you can perform a faster version of the validation by using the FAST clause. This version checks for the existence of corruptions using an optimized check algorithm, but does not report details about the corruption. If the FAST check finds a corruption, you can then use the CASCADE option without the FAST clause to locate it. The following statement performs a fast validation on the emp table and all associated indexes:

ANALYZE TABLE emp VALIDATE STRUCTURE CASCADE FAST;

You can specify that you want to perform structure validation online while DML is occurring against the object being validated. There can be a slight performance impact when validating with ongoing DML affecting the object, but this is offset by the flexibility of being able to perform ANALYZE online. The following statement validates the emp table and all associated indexes online:

ANALYZE TABLE emp VALIDATE STRUCTURE CASCADE ONLINE;

See Also:

Oracle Database SQL Language Reference for more information on the ANALYZE statement

Listing Chained Rows of Tables and Clusters

You can look at the chained and migrated rows of a table or cluster using the ANALYZE statement with the LIST CHAINED ROWS clause. The results of this statement are stored in a specified table created explicitly to accept the information returned by the LIST CHAINED ROWS clause. These results are useful in determining whether you have enough room for updates to rows.

Creating a CHAINED_ROWS Table

To create the table to accept data returned by an ANALYZE...LIST CHAINED ROWS statement, execute the UTLCHAIN.SQL or UTLCHN1.SQL script. These scripts are provided by the database. They create a table named CHAINED_ROWS in the schema of the user submitting the script.

Note:

Your choice of script to execute for creating the CHAINED_ROWS table is dependent upon the compatibility level of your database and the type of table you are analyzing. See the Oracle Database SQL Language Reference for more information.

After a CHAINED_ROWS table is created, you specify it in the INTO clause of the ANALYZE statement. For example, the following statement inserts rows containing information about the chained rows in the emp_dept cluster into the CHAINED_ROWS table:

ANALYZE CLUSTER emp_dept LIST CHAINED ROWS INTO CHAINED_ROWS;

See Also:

• Oracle Database Reference for a description of the CHAINED_ROWS table
  
• "Using the Segment Advisor" for information on how the Segment Advisor reports tables with excess row chaining.
  

Eliminating Migrated or Chained Rows in a Table

You can use the information in the CHAINED_ROWS table to reduce or eliminate migrated and chained rows in an existing table. Use the following procedure.

1. Use the ANALYZE statement to collect information about migrated and chained rows.
   

   ANALYZE TABLE order_hist LIST CHAINED ROWS;
   

2. Query the output table:
   

   SELECT *
   FROM CHAINED_ROWS
   WHERE TABLE_NAME = 'ORDER_HIST';
   
   OWNER_NAME  TABLE_NAME  CLUST... HEAD_ROWID          TIMESTAMP
   ----------  ----------  -----... ------------------  ---------
   SCOTT       ORDER_HIST       ... AAAAluAAHAAAAA1AAA  04-MAR-96
   SCOTT       ORDER_HIST       ... AAAAluAAHAAAAA1AAB  04-MAR-96
   SCOTT       ORDER_HIST       ... AAAAluAAHAAAAA1AAC  04-MAR-96
   

   The output lists all rows that are either migrated or chained.
   
3. If the output table shows that you have many migrated or chained rows, then you can eliminate migrated rows by continuing through the following steps:
   
4. Create an intermediate table with the same columns as the existing table to hold the migrated and chained rows:
   

   CREATE TABLE int_order_hist
      AS SELECT *
         FROM order_hist
         WHERE ROWID IN
            (SELECT HEAD_ROWID
               FROM CHAINED_ROWS
               WHERE TABLE_NAME = 'ORDER_HIST');
   

5. Delete the migrated and chained rows from the existing table:
   

   DELETE FROM order_hist
      WHERE ROWID IN
         (SELECT HEAD_ROWID
            FROM CHAINED_ROWS
            WHERE TABLE_NAME = 'ORDER_HIST');
   

6. Insert the rows of the intermediate table into the existing table:
   

   INSERT INTO order_hist
      SELECT *
      FROM int_order_hist;
   

7. Drop the intermediate table:
   

   DROP TABLE int_order_history;
   

8. Delete the information collected in step 1 from the output table:
   

   DELETE FROM CHAINED_ROWS
      WHERE TABLE_NAME = 'ORDER_HIST';
   

9. Use the ANALYZE statement again, and query the output table.
   

Any rows that appear in the output table are chained. You can eliminate chained rows only by increasing your data block size. It might not be possible to avoid chaining in all situations. Chaining is often unavoidable with tables that have a LONG column or large CHAR or VARCHAR2 columns.