Difference between DBA_TABLES.num_rows and count(*)

One thing DBAs regularly do is to gather information on the schemas in the database for reporting. So we want to know how many rows there are in the tables of the schema “SIMON”. When doing so, one is tempted to query the DBA_TABLES view:

SQL> SELECT table_name, tablespace_name, num_rows
  2  FROM dba_tables WHERE owner='SIMON' ORDER BY num_rows DESC;

TABLE_NAME                     TABLESPACE_NAME                  NUM_ROWS
------------------------------ ------------------------------ ----------
HUGE_TABLE                     DZ_DATA01                         8227990

However, if we query the table directly, we get a different result:

COUNT(*) ---------- 8230310

Why is that so? When reading through the description of the ALL_TABLES view, one finds the following note:

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Hello world

My name is Simon Krenger, I am a Technical Account Manager (TAM) at Red Hat. I advise our customers in using Kubernetes, Containers, Linux and Open Source.


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