Routinely Maintaining Tables

To ensure proper database running, after INSERT and DELETE operations, you need to routinely do VACUUM FULL and ANALYZE as appropriate for customer scenarios and update statistics to obtain better performance.

Related Concepts

You need to routinely run VACUUM, VACUUM FULL, and ANALYZE to maintain tables, because:

• VACUUM FULL reclaims disk space occupied by updated or deleted data and combines small-size data files.

• VACUUM maintains a visualized mapping to track pages that contain arrays visible to other active transactions. A common index scan uses the mapping to obtain the corresponding array and check whether pages are visible to the current transaction. If the array cannot be obtained, the visibility is checked by fetching stack arrays. Therefore, updating the visible mapping of a table can accelerate unique index scans.

• VACUUM can avoid old data loss caused by duplicate transaction IDs when the number of executed transactions exceeds the database threshold.

• ANALYZE collects statistics on tables in databases. The statistics are stored in the PG_STATISTIC system catalog. Then, the query optimizer uses the statistics to work out the most efficient execution plan.

Procedure

1. Run the VACUUM or VACUUM FULL command to reclaim disk space.

   • VACUUM:

     Do VACUUM to the table:

     VACUUM customer;
     

     VACUUM
     

     This command can be concurrently executed with database operation commands, including SELECT, INSERT, UPDATE, and DELETE; excluding ALTER TABLE.

     Do VACUUM to the partitioned table:

     VACUUM customer_par PARTITION ( P1 );
     

     VACUUM
     

   • VACUUM FULL:

     VACUUM FULL customer;
     

     VACUUM
     

     VACUUM FULL needs to add exclusive locks on tables it operates on and requires that all other database operations be suspended.

     When reclaiming disk space, you can query for the session corresponding to the earliest transactions in the cluster, and then end the earliest long transactions as needed to make full use of the disk space.

     1. Run the following command to query for oldestxmin on the GTM:

        select * from pgxc_gtm_snapshot_status();
        

     2. Run the following command to query for the PID of the corresponding session on the CN. xmin is the oldestxmin obtained in the previous step.

        select * from pgxc_running_xacts() where xmin=1400202010;
        

2. Do ANALYZE to update statistical information.

   ANALYZE customer;
   

   ANALYZE
   

   Do ANALYZE VERBOSE to update statistics and display table information.

   ANALYZE VERBOSE customer;
   

   ANALYZE
   

   You can use VACUUM ANALYZE at the same time to optimize the query.

   VACUUM ANALYZE customer;
   

   VACUUM
   

   Note

   VACUUM and ANALYZE cause a substantial increase in I/O traffic, which may cause poor performance of other active sessions. Therefore, you are advised to set by specifying the vacuum_cost_delay parameter.

3. Delete a table

   DROP TABLE customer;
   DROP TABLE customer_par;
   DROP TABLE part;
   

   If the following output is displayed, the index has been deleted.

   DROP TABLE
   

Maintenance Suggestion

• Routinely do VACUUM FULL to large tables. If the database performance deteriorates, do VACUUM FULL to the entire database. If the database performance is stable, you are advised to monthly do VACUUM FULL.

• Routinely do VACUUM FULL to system catalogs, mainly PG_ATTRIBUTE.

• The automatic vacuum process (AUTOVACUUM) in the system automatically runs the VACUUM and ANALYZE statements to reclaim the record space marked as the deleted state and to update statistics related to the table.

last updated: 2024-11-13 15:46 UTC - commit: bdcb01d62e4f1de672b662d88ef6cb8eb3a27a29