What is EXPLAIN in the context of working with…

EXPLAIN is an SQL operator that allows seeing how the database management system (DBMS) plans to execute an SQL query. It shows the execution plan of the query, including the order of operations (table scans, joins, sorts, index usage, etc.) and the estimated number of rows processed.

Analyzing the output of EXPLAIN helps to understand:

Are indexes used: If not, it may indicate the need to create them.
Join order of tables: A non-optimal order can slow down the query.
Full table scan: If the query scans the entire table instead of using an index, it could be a performance issue.
Additional operations: Sorting (ORDER BY) or aggregation without the appropriate index can be resource-intensive.

The syntax and output of EXPLAIN may vary slightly across different DBMSs (PostgreSQL, MySQL, SQLite, etc.), but the overall goal — showing the execution plan — remains the same.

Example usage in PostgreSQL:

EXPLAIN SELECT customer_id, order_date
FROM orders
WHERE order_date > '2023-01-01'
ORDER BY order_date;

A simplified example output might look like this:

Sort  (cost=123.45..123.50 rows=10 width=12)
  ->  Index Scan using orders_date_idx on orders  (cost=0.56..123.40 rows=10 width=12)
        Index Cond: (order_date > '2023-01-01'::date)

This output shows that PostgreSQL plans to use the index orders_date_idx for filtering by date, then perform sorting of the results.

In MySQL, the syntax is similar:

EXPLAIN SELECT customer_id, order_date
FROM orders
WHERE order_date > '2023-01-01'
ORDER BY order_date;

MySQL's output is usually presented as a table with columns like id, select_type, table, partitions, type, possible_keys, key, key_len, ref, rows, filtered, Extra.

type – one of the most important columns, showing how MySQL searches for rows in the table (e.g., ALL - full table scan, index - by index, ref - by non-unique index, eq_ref - by unique index).
key – the index used.
rows – an approximate number of rows that MySQL plans to examine.

By analyzing the output of EXPLAIN, decisions can be made regarding optimization, such as adding or modifying indexes, rewriting queries, or changing data structures.

What is EXPLAIN in the context of working with databases?