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How to Update Table Statistics In Oracle?

6 minutes read

In Oracle, you can update table statistics by using the DBMS_STATS package. This package provides procedures for gathering and updating statistics for tables in the database. To update table statistics, you can use the "GATHER_TABLE_STATS" procedure from the DBMS_STATS package. This procedure allows you to gather or update statistics for a specific table or schema. You can specify various options such as estimate_percent, method_opt, and cascade when gathering statistics for a table. Updating table statistics is important for the query optimizer to generate efficient execution plans for SQL queries. It helps the optimizer to make informed decisions about index usage, access paths, and join methods. By updating table statistics regularly, you can ensure that the database optimizer has accurate information about the data distribution and cardinality of tables in the database.


How to update index statistics in Oracle?

To update index statistics in Oracle, you can use the DBMS_STATS package. Here are the steps to do this:

  1. Connect to your Oracle database using SQL*Plus or a similar tool.
  2. Run the following command to gather statistics for a specific index: 
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EXEC DBMS_STATS.GATHER_INDEX_STATS('schema_name', 'index_name');


Replace 'schema_name' with the schema name where the index is located and 'index_name' with the name of the index for which you want to update statistics.

  1. If you want to gather statistics for all indexes in a schema, you can use the following command:
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EXEC DBMS_STATS.GATHER_SCHEMA_STATS('schema_name');


Replace 'schema_name' with the name of the schema for which you want to gather statistics.

  1. You can also gather statistics for a specific table that the index is associated with using the following command:
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EXEC DBMS_STATS.GATHER_TABLE_STATS('schema_name', 'table_name');


Replace 'schema_name' with the schema name and 'table_name' with the name of the table associated with the index.

  1. After running the above commands, the index statistics will be updated and the query optimizer will use the updated statistics for query execution.


Please note that updating index statistics can potentially impact the performance of your database queries, so it is recommended to gather statistics during off-peak hours. Additionally, always back up your database before performing any maintenance tasks.


How to update table statistics in Oracle with a PL/SQL script?

To update table statistics in Oracle using PL/SQL, you can use the DBMS_STATS package. Here is an example of a PL/SQL script that updates statistics for a specific table:

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DECLARE
   ownname VARCHAR2(30) := 'SCHEMA_OWNER';  -- Replace with the schema owner name
   tabname VARCHAR2(30) := 'TABLE_NAME';  -- Replace with the table name
BEGIN
   DBMS_STATS.GATHER_TABLE_STATS(ownname, tabname);
END;
/


Make sure to replace 'SCHEMA_OWNER' and 'TABLE_NAME' with the actual schema owner and table name that you want to update statistics for. This script will gather fresh statistics for the specified table, which can help the Oracle optimizer make better execution plan decisions.


How to identify stale statistics in Oracle?

Stale statistics in Oracle can lead to inefficient query performance and inaccurate optimization decisions by the query optimizer. To identify stale statistics in Oracle, you can follow these steps:

  1. Use the DBMS_STATS package to gather statistics for all tables and indexes in the database:
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EXEC DBMS_STATS.GATHER_DATABASE_STATS;


  1. Check the last analyzed date for tables and indexes in the USER_TABLES and USER_INDEXES views:
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SELECT table_name, last_analyzed 
FROM user_tables 
WHERE last_analyzed < SYSDATE - 30; -- Check for tables not analyzed in last 30 days

SELECT index_name, last_analyzed
FROM user_indexes 
WHERE last_analyzed < SYSDATE - 30; -- Check for indexes not analyzed in last 30 days


  1. Use the DBA_TAB_STATISTICS and DBA_IND_STATISTICS views to check the number of rows in tables or indexes compared to the number of rows estimated by the optimizer:
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SELECT table_name, num_rows, sample_size 
FROM DBA_TAB_STATISTICS 
WHERE num_rows <> sample_size;

SELECT table_name, index_name, num_rows, sample_size 
FROM DBA_IND_STATISTICS 
WHERE num_rows <> sample_size;


  1. Monitor query performance and look for any sudden degradation in performance that may indicate stale statistics.
  2. Use the DBMS_STATS package to manually gather statistics for specific tables or indexes that are identified as stale: 
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EXEC DBMS_STATS.GATHER_TABLE_STATS('schema_name', 'table_name');
EXEC DBMS_STATS.GATHER_INDEX_STATS('schema_name', 'index_name');


By regularly monitoring and gathering statistics for tables and indexes in Oracle, you can ensure that the query optimizer makes accurate decisions and improves overall database performance.


What is the difference between estimate_percent and block_sample in Oracle table statistics?

The difference between estimate_percent and block_sample in Oracle table statistics lies in the method they use to gather statistics for a table:

  1. estimate_percent: This method involves estimating the percentage of rows in a table that need to be sampled in order to generate accurate statistics. The optimizer then uses this estimated percentage to sample the table and gather statistics. This method is useful when you want to quickly gather statistics for a large table without sampling the entire table, as it can be more efficient in terms of time and resources.
  2. block_sample: This method involves sampling a specified number of database blocks from a table to gather statistics. The blocks are chosen randomly from the table, and the optimizer uses the data in these blocks to generate statistics. This method is useful when you want to gather statistics that are more accurate and representative of the entire table, as it samples actual data blocks rather than estimating a percentage of rows to sample.


In summary, estimate_percent estimates the percentage of rows to sample, while block_sample samples actual data blocks from the table to gather statistics. The choice between the two methods depends on the specific requirements and constraints of your database environment.


What is the best way to gather statistics for a large table in Oracle?

The best way to gather statistics for a large table in Oracle is to use the DBMS_STATS package to collect and update statistics on the table. This package provides procedures and functions to gather statistics on schema objects such as tables, indexes, and columns.


Some common ways to gather statistics for a large table using the DBMS_STATS package are:

  1. Gathering statistics on the table using the GATHER_TABLE_STATS procedure:
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EXEC DBMS_STATS.GATHER_TABLE_STATS('schema_name', 'table_name');


  1. Gathering statistics on the table and its indexes using the GATHER_SCHEMA_STATS procedure:
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EXEC DBMS_STATS.GATHER_SCHEMA_STATS('schema_name', 'table_name');


  1. Gathering statistics on columns of the table using the GATHER_COLUMN_STATS procedure:
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EXEC DBMS_STATS.GATHER_COLUMN_STATS('schema_name', 'table_name', 'column_name');


By using the DBMS_STATS package to gather statistics for a large table, you can ensure that the optimizer has accurate and up-to-date information to generate optimal execution plans for queries involving the table.


How to update table statistics for a specific table in Oracle?

You can update table statistics for a specific table in Oracle using the DBMS_STATS package. Here is an example of how you can do this:

  1. Connect to your Oracle database using a SQL tool like SQL*Plus or SQL Developer.
  2. Run the following PL/SQL code to gather statistics for a specific table named 'my_table': 
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BEGIN
  DBMS_STATS.GATHER_TABLE_STATS(
    ownname => 'your_schema',
    tabname => 'my_table',
    estimate_percent => DBMS_STATS.AUTO_SAMPLE_SIZE,
    method_opt => 'FOR ALL COLUMNS SIZE AUTO'
  );
END;
/


In the code snippet above:

  • Replace 'your_schema' with the name of the schema that contains the table.
  • Replace 'my_table' with the name of the table for which you want to gather statistics.
  • The estimate_percent parameter specifies the percentage of rows to sample when gathering statistics. Using DBMS_STATS.AUTO_SAMPLE_SIZE will let Oracle determine the appropriate sample size automatically.
  • The method_opt parameter specifies how Oracle should gather statistics for all columns in the table.
  1. After running the code, Oracle will gather statistics for the specified table, which will help the query optimizer generate efficient execution plans for queries involving this table.


Note: It is recommended to gather statistics regularly on important tables to ensure accurate query performance.

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