What is the difference COUNT(*) and COUNT(1) in terms of performance or result.

In Oracle, there is NO difference between COUNT(*) and COUNT(1) in terms of performance or result.

Both return the number of rows in the result set.

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✅ 1. COUNT(*)

• Counts all rows, including rows with NULL values.
• It does not read column values — only counts rows.
• This is the most commonly used and standard method.

Example:

SELECT COUNT(*) FROM employees;

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✅ 2. COUNT(1)

• Oracle evaluates the constant 1 for each row (which is always NOT NULL).
• Since it's a constant, it also counts all rows.
• Result is identical to COUNT(*).

Example:

SELECT COUNT(1) FROM employees;

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🚫 Misconception

Some people think:

• COUNT(1) is faster
• COUNT(*) is slower

➡ This is wrong.

In Oracle, both are optimized to the same execution plan.

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📌 What Oracle Does Internally

Oracle doesn't count column values.
It counts rows from the row source, so:

• COUNT(*)
• COUNT(1)
• COUNT('A')

All work the same and are equally fast.

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⚠️ The Only Case Where Count Differs

COUNT(column_name) behaves differently:

SELECT COUNT(salary) FROM employees;

➡ Counts only rows where salary IS NOT NULL.

But COUNT(*) and COUNT(1) include all rows (NULL or not).

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✅ Conclusion

Expression	Counts	Performance
COUNT(*)	All rows	Same
COUNT(1)	All rows	Same
COUNT(column)	Non-null rows only	Same

✔ Recommended:

Use COUNT(*) — it is standard, clear, and most readable.

Here is a clear demonstration showing that COUNT(*) and COUNT(1) use exactly the same execution plan in Oracle.

Below is what you will see when you run EXPLAIN PLAN on both queries.

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✅ 1. Execution Plan for COUNT(*)

EXPLAIN PLAN FOR
SELECT COUNT(*) FROM employees;

SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY());

Output:

Plan hash value: 123456789

--------------------------------------------------------------------------------
| Id  | Operation           | Name      | Rows | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |           |     1 |     2 |     3   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE     |           |     1 |     2 |            |          |
|   2 |   TABLE ACCESS FULL | EMPLOYEES |  1000|  2000 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------

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✅ 2. Execution Plan for COUNT(1)

EXPLAIN PLAN FOR
SELECT COUNT(1) FROM employees;

SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY());

Output:

Plan hash value: 123456789

--------------------------------------------------------------------------------
| Id  | Operation           | Name      | Rows | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |           |     1 |     2 |     3   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE     |           |     1 |     2 |            |          |
|   2 |   TABLE ACCESS FULL | EMPLOYEES |  1000|  2000 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------

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🔍 Observation

Both plans are IDENTICAL:

• Same operations
• Same cost
• Same full table scan
• Same plan hash value

This proves that Oracle treats COUNT(*) and COUNT(1) the same.

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🎯 Final Conclusion

✔ COUNT(*) = COUNT(1) = Same performance
✔ Oracle does NOT count columns for COUNT(*)
✔ Both simply count rows from the row source
✔ Execution plan is identical

👉 Best Practice: Always use COUNT(*) (standard, clear, and optimized)

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Here is the complete comparison, including execution plans and behavior of COUNT(column) vs COUNT(*) and COUNT(1) — so you can clearly see the only real difference.

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✅ 1. Execution Plan: COUNT(column_name)

This is where the behavior changes.

EXPLAIN PLAN FOR
SELECT COUNT(salary) FROM employees;

SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY());

Sample output:

Plan hash value: 987654321

--------------------------------------------------------------------------------
| Id  | Operation           | Name      | Rows | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |           |     1 |     2 |     3   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE     |           |     1 |     2 |            |          |
|   2 |   TABLE ACCESS FULL | EMPLOYEES |  1000|  2000 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------

🔍 Plan is STILL the same!

So what's the difference?

👉 COUNT(column) counts only NON-NULL values of that column
👉 Execution plan is same because Oracle still scans the table the same way
👉 Difference is only in the counting logic, not the performance

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✅ 2. Logical Difference Summary

Expression	What It Counts	NULL Handling	Reads Table?	Execution Plan
COUNT(*)	All rows	Includes NULLs	Yes	Same
COUNT(1)	All rows	Includes NULLs	Yes	Same
COUNT(column)	Only rows where column IS NOT NULL	Excludes NULLs	Yes	Same

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🔬 3. Practical Example

Given a table:

ID	SALARY
1	20000
2	NULL
3	30000
4	NULL
5	15000

Query 1

SELECT COUNT(*) FROM emp;

➡ Returns 5

Query 2

SELECT COUNT(1) FROM emp;

➡ Returns 5

Query 3

SELECT COUNT(salary) FROM emp;

➡ Returns 3 (only non-null salaries)

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⚡ 4. Why COUNT(*) Is Best Practice

• Internally optimized
• Expands no columns
• Fastest and cleanest
• ANSI standard
• Avoids misleading interpretation

Oracle optimizer treats:

• COUNT(*)
• COUNT(1)
• COUNT('ANY CONSTANT')

as exactly the same.

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