DBMS_STATS.GATHER_TABLE_STATS如何在Oracle中工作

Question

I need clarification on how DBMS_STATS.GATHER_TABLE_STATS works. I have a scenario,were i am creating table and the same table is used in the package creation also.In other words the package compilation is depended on table creation.

Is it mandatory to include DBMS_STATS.GATHER_TABLE_STATS after index creation command in table creation script.

In which situation DBMS_STATS.GATHER_TABLE_STATS works,whether it's for package compilation or package execution.Please confirm.

Answer 1

DBMS_STATS provides information to the Oracle optimizer, enabling Oracle to build efficient execution plans for SQL statements. Optimizer statistics are never necessary for compiling objects.

Gathering statistics is such a complicated subject it can be difficult to even ask the right questions at first. I'd recommend starting by reading the manual, such as Managing Optimizer Statistics: Basic Topics .

Typically your statistics gathering strategy should look something like this:

1. Gather statistics manually whenever an object's data significantly changes.
2. Use the default autotask to automatically gather statistics for objects that have been slowly changing or were accidentally forgotten.

To answer some specific questions, you probably do not want to gather statistics right after creating a table. Only gather statistics after changing the data. Gathering statistics on an empty table can be dangerous; Oracle will think the table has 0 rows until the next time statistics are gathered. It would be better to have no statistics at all, then Oracle can use dynamic sampling to estimate the statistics.

(But on the other hand, if the table is created as a SELECT statement and contains a lot of data you may want to gather statistics. Then again, if you're using 12c it might gather statistics automatically when creating a table.)

You also generally do not need to gather statistics after creating an index. Oracle automatically gathers index statistics when an index is created or rebuilt.