Faster alternative in Oracle to SELECT COUNT(*) FROM sometable

Question

I've notice that in Oracle, the query

SELECT COUNT(*) FROM sometable;

is very slow for large tables. It seems like the database it actually going through every row and incrementing a counter one at a time. I would think that there would be a counter somewhere in the table how many rows that table has.

So if I want to check the number of rows in a table in Oracle, what is the fastest way to do that?

Answer 1

If you want just a rough estimate, you can extrapolate from a sample:

SELECT COUNT(*) * 100 FROM sometable SAMPLE (1);

For greater speed (but lower accuracy) you can reduce the sample size:

SELECT COUNT(*) * 1000 FROM sometable SAMPLE (0.1);

For even greater speed (but even worse accuracy) you can use block-wise sampling:

SELECT COUNT(*) * 100 FROM sometable SAMPLE BLOCK (1);

Answer 2

This works great for large tables.

SELECT NUM_ROWS FROM ALL_TABLES WHERE TABLE_NAME = 'TABLE_NAME_IN_UPPERCASE';

For small to medium size tables, following will be ok.

SELECT COUNT(Primary_Key) FROM table_name;

Cheers,

Answer 3

Think about it: the database really has to go to every row to do that. In a multi-user environment my COUNT(*) could be different from your COUNT(*). It would be impractical to have a different counter for each and every session so you have literally to count the rows. Most of the time anyway you would have a WHERE clause or a JOIN in your query so your hypothetical counter would be of litte practical value.

There are ways to speed up things however: if you have an INDEX on a NOT NULL column Oracle will count the rows of the index instead of the table. In a proper relational model all tables have a primary key so the COUNT(*) will use the index of the primary key.

Bitmap index have entries for NULL rows so a COUNT(*) will use a bitmap index if there is one available.