The difference between count (*), COUNT (1), and count (primary key)

Source: Internet
Author: User

If NULL participates in the aggregation operation, all other aggregation functions except count (*) ignore null. Such as:


1 E


Select COUNT (*) from table--the result is 2

Select COUNT (DD) from table---result is 1

Efficiency issues:

When the table has a larger amount of data, using count (1) is more than using count (*) when analyzing the table.

The effect of count (1) and COUNT (*) is the same from the execution plan.

However, after the table has been analyzed, COUNT (1) will be less than COUNT (*) (within 1w of data), but not much.

This is also related to the number of records in the table! If the amount of data outside of 1w has been analyzed by the table, the Count (1) is more than count (*).

Also, using count (1) is a little bit less than using count (*) when the amount of data reaches more than 10w.

If your data table does not have a primary key, then count (1) is faster than COUNT (*)

If there is a primary key, then the primary key (the Federated primary key) as the count condition is also faster than COUNT (*)

If your table has only one field, then count (*) is the quickest.

COUNT (*) count (1) compares the two. The data field that corresponds to count (1) is mostly still.

If Count (1) is a clustered index, ID, that must be count (1) fast. But the difference is very small.

Because Count (*), automatically optimizes the specified field to that one. So no need to go to count (1), with Count (*), SQL will help you to complete the optimized

So: Count (1) and COUNT (*) are basically no different!

SQL tuning is primarily about reducing the number of consistent gets and physical reads.

The difference between count (*), COUNT (1), and count (primary key)

Contact Us

The content source of this page is from Internet, which doesn't represent Alibaba Cloud's opinion; products and services mentioned on that page don't have any relationship with Alibaba Cloud. If the content of the page makes you feel confusing, please write us an email, we will handle the problem within 5 days after receiving your email.

If you find any instances of plagiarism from the community, please send an email to: and provide relevant evidence. A staff member will contact you within 5 working days.

A Free Trial That Lets You Build Big!

Start building with 50+ products and up to 12 months usage for Elastic Compute Service

  • Sales Support

    1 on 1 presale consultation

  • After-Sales Support

    24/7 Technical Support 6 Free Tickets per Quarter Faster Response

  • Alibaba Cloud offers highly flexible support services tailored to meet your exact needs.