Why Does 'explain' Report a Different Row Count Than 'count()'?

Discrepancy in Row Counts Returned by "explain" and "count()": A Closer Look

A puzzling situation arises when comparing the number of rows returned by "explain" and the actual row count obtained from "count()". While it may seem intuitive to assume that "explain" provides an accurate estimate of the number of rows fetched, this is not always the case.

Understanding the "explain" Command

The "explain" command doesn't actually execute the query; instead, it analyzes the query plan and estimates the number of rows that will be processed. This estimate is based on various factors, including statistics maintained about the tables involved.

Why the Rows Reported by "explain" May Differ

The discrepancy between the "explain" output and the actual row count can be due to the following reasons:

• Estimation inaccuracies: The estimates used by "explain" are not always precise, especially for complex queries involving multiple tables or joins.
• Outdated statistics: The statistics upon which "explain" relies may not be up-to-date, leading to inaccurate estimates.

An Example of the Discrepancy

Consider the following example:

mysql> select count(*) from table where relation_title='xxxxxxxxx';
+----------+
| count(*) |
+----------+
|  1291958 |
+----------+
mysql> explain select *  from table where relation_title='xxxxxxxxx';
+----+-------------+---------+-
| id | select_type | rows    |
+----+-------------+---------+-
|  1 | SIMPLE      | 1274785 | 
+----+-------------+---------+-

As you can see, "explain" estimates that the query will process 1274785 rows, while "count(*)" returns a count of 1291958 rows. This discrepancy arises due to the estimation inaccuracies and outdated statistics explained above.

Importance of Accurate Row Counts

Obtaining accurate row counts is crucial for optimizing queries and avoiding unnecessary overhead. By understanding the limitations of "explain," developers can make informed decisions about whether to rely on its estimates or seek alternative methods for determining the number of rows processed.

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