Table Statistics Gathering Query

Question

Hey there,
I'm currently getting trained in Oracle and one of the questions posed to me were create a table, insert a million rows into it and try to find the number of rows in it. I've tried the following steps to solve this,
First table creation
SQL> create table t1(id number);
Table created.Data insertion
SQL> insert into t1 select level from dual connect by level < 50000000;
49999999 rows created.Gathering statistics
SQL> exec dbms_stats.gather_table_stats('HR','T1');
PL/SQL procedure successfully completed.Finally counting the number of rows
SQL> select num_rows from user_tables where table_name='T1';
  NUM_ROWS
  49960410
SQL> select count(*) from t1;
  COUNT(*)
  49999999My database version is,
SQL> select * from v$version;
BANNER
Oracle Database 10g Express Edition Release 10.2.0.1.0 - Product
PL/SQL Release 10.2.0.1.0 - Production
CORE    10.2.0.1.0      Production
TNS for 32-bit Windows: Version 10.2.0.1.0 - Production
NLSRTL Version 10.2.0.1.0 - ProductionI would like to know why there are two different results for the same table when using "num_rows" from the view "user_tables" and the aggregate function "count()" over the same table. Please do keep in mind that i'm studying oracle and this is from a conceptual point of view only. I would like to know how gathering the table statistics works using dbms_stats package works.
Thank You,
Vishal

Answer 1

vishm8 wrote:
Gathering statistics
SQL> exec dbms_stats.gather_table_stats('HR','T1');
PL/SQL procedure successfully completed.I would like to know why there are two different results for the same table when using "num_rows" from the view "user_tables" and the aggregate function "count()" over the same table. Please do keep in mind that i'm studying oracle and this is from a conceptual point of view only. I would like to know how gathering the table statistics works using dbms_stats package works.
Thank You,
VishalBecause you aren't specifying a value for estimate_percent in the procedure call (to gather_table_stats) Oracle will pick an estimate value for you. If you want to sample the entire table you would need to explicitly specify that in your procedure call.
http://download.oracle.com/docs/cd/E11882_01/appdev.112/e16760/d_stats.htm#ARPLS68582