What is SQL_BIG_SELECTS and how can I avoid encountering it in MySQL?

Understanding SQL_BIG_SELECTS in MySQL

SQL_BIG_SELECTS is a MySQL optimization parameter designed to prevent extremely large and potentially problematic SELECT queries from executing. When a query is deemed "too large," MySQL may return an error similar to:

ERROR 1104: The SELECT would examine too many records and probably take a very long time.
Check your WHERE and use SET OPTION SQL_BIG_SELECTS=1 if the SELECT is ok

To effectively address this issue, let's explore the answers to specific questions:

1. Threshold for "BIG SELECTS"

MySQL determines a "BIG SELECT" based on the value of the 'max_join_size' system variable. If a query is likely to examine more than this number of rows, it is flagged as such. Use 'show variables' to view the current max join size setting.

2. Indexing and Big SELECTs

Proper indexing and an efficient WHERE clause can mitigate the risk of encountering a SQL_BIG_SELECTS error. Indexes help speed up query execution, reducing the number of rows examined. A well-defined WHERE clause can further reduce the scope of the query and avoid scanning unnecessary data.

3. SQL_BIG_SELECTS as a Last Resort

SQL_BIG_SELECTS should not be used as a general solution. It is intended as an emergency measure to prevent excessively large queries from consuming system resources. It is good practice to avoid setting it globally or for all users.

4. Configuring SQL_BIG_SELECTS

To set SQL_BIG_SELECTS, you can:

• Modify the my.cnf configuration file and add "sql-big-selects=ON" under the [mysqld] section.
• Set it at server startup using the --sql-big-selects command-line option.
• Set it on a session basis using the query "SET SESSION SQL_BIG_SELECTS=1".

5. Alternatives to SQL_BIG_SELECTS

Consider the following alternatives before resorting to SQL_BIG_SELECTS:

• Optimize your queries with proper indexing and WHERE clauses.
• Increase the max_join_size system variable to allow larger queries.

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