Most of the gains in performance derive from good database design, thorough query analysis, and appropriate indexing. The largest performance gains can be realized by establishing a good database design.
1. Update Table Statistics in database.
SYBASE SYNTAX: update all statistics table_name
Adaptive Server’s cost-based optimizer uses statistics about the tables, indexes, and columns named in a query to estimate query costs. It chooses the access method that the optimizer determines has the least cost. But this cost estimate cannot be accurate if statistics are not accurate. Some statistics, such as the number of pages or rows in a table, are updated during query processing. Other statistics, such as the histograms on
columns, are only updated when you run the update statistics command or when indexes are created.
If you are having problems with a query performing slowly, and seek help from Technical Support or a Sybase news group on the Internet, one of the first questions you are likely be asked is “Did you run update statistics?” You can use the optdiag command (IN SYBASE) to see the time update statistics was last run for each column on which statistics exist:
NOTE:
• Running the update statistics commands requires system resources. Like other maintenance tasks, it should be scheduled at times when load on the server is light. In particular, update statistics requires table scans or leaf-level scans of indexes, may increase I/O contention, may use the CPU to perform sorts, and uses the data and procedure caches. Use of these resources can adversely affect queries running on the server if you run update statistics at times when usage is high. In addition, some update statistics commands require shared locks, which can block updates.
• Dropping an index does not drop the statistics for the index, since the optimizer can use column-level statistics to estimate costs, even when no index exists. If you want to remove the statistics after dropping an index, you must explicitly delete them with delete statistics.
• Truncating a table does not delete the column-level statistics in sysstatistics. In many cases, tables are truncated and the same data is reloaded. Since truncate table does not delete the column-level statistics, there is no need to run update statistics after the table is reloaded, if the data is the same. If you reload the table with data that has a different distribution of key values, you need to run update statistics.
• You can drop and re-create indexes without affecting the index statistics, by specifying 0 for the number of steps in the with statistics clause to create index. This create index command does not affect the statistics in sysstatistics (IN SYBASE):
Create index title_id_ix on titles (title_id) with statistics using 0 values
This allows you to re-create an index without overwriting statistics that have been edited with optdiag.
• If two users attempt to create an index on the same table, with the same columns, at the same time, one of the commands may fail due to an attempt to enter a duplicate key value in sysstatistics.
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