Why Does "explain" Row Count Differ From "count()" Result in MySQL?

Discrepancy Between "explain" and "count()" Results: Exploring Potential Causes

In MySQL, the "explain" command can be used to analyze query performance and estimate the number of rows processed during the query execution. However, in some cases, the row count reported by "explain" may not match the actual number of rows returned by "count()".

Why does this discrepancy occur?

The row count displayed by "explain" represents an estimate of the number of rows that the query optimizer believes will be accessed during query execution. This estimate is based on statistical information about the table and its indexes. It is important to note that the actual number of rows processed may vary from the estimate.

Potential causes for the discrepancy:

• Index usage: "explain" assumes that indexes will be used to efficiently access the data. However, if the query does not use an optimal index or if the index is not up-to-date, the actual row count may be higher.
• Data distribution: The "explain" estimate is based on the distribution of data in the table. If the data is skewed, the actual row count may differ from the estimate.
• Updated data: Since "explain" relies on statistical information, it is not aware of any changes made to the data since the statistics were last updated. If the data has been significantly modified, the actual row count may be different from the estimate.
• Inaccurate cardinality estimates: The optimizer may use estimated cardinalities for tables and indexes, which can lead to inaccurate row count estimates.

Importance of the discrepancy:

Understanding the discrepancy between "explain" and "count()" is crucial for accurately assessing query performance. A significant difference in the row counts can indicate that the query is not using optimal indexes or that the statistical information is inaccurate.

Conclusion:

The "explain" command provides valuable insights into query execution but should be used cautiously when determining the exact number of rows processed. By being aware of the potential causes for discrepancy between "explain" and "count()", you can optimize your queries and ensure accurate performance analysis.

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