Why used "explain" in MYSQL ?

Explain

• The EXPLAIN statement provides information about how MySQL executes statements
• works with SELECT, DELETE, INSERT, REPLACE, and UPDATE statements.
• is used with an explainable statement, MySQL displays information from the optimizer about the statement execution plan.

How to used Explain

• This section describes the output columns produced by EXPLAIN. Later sections provide additional information about the type and Extra columns.

EXPLAIN Join Types

The type column of EXPLAIN output describes how tables are joined. In JSON-formatted output, these are found as values of the access_type property. The following list describes the join types, ordered from the best type to the worst:

system

The table has only one row (= system table). This is a special case of the const join type.

const

The table has at most one matching row, which is read at the start of the query. Because there is only one row, values from the column in this row can be regarded as constants by the rest of the optimizer. const tables are very fast because they are read only once.

const is used when you compare all parts of a PRIMARY KEY or UNIQUE index to constant values. In the following queries, tbl_name can be used as a const table

   `SELECT * FROM tbl_name WHERE primary_key=1;

   SELECT * FROM tbl_name
    WHERE primary_key_part1=1 AND primary_key_part2=2;`

eq_ref

One row is read from this table for each combination of rows from the previous tables. Other than the system and const types, this is the best possible join type. It is used when all parts of an index are used by the join and the index is a PRIMARY KEY or UNIQUE NOT NULL index.

eq_ref can be used for indexed columns that are compared using the = operator. The comparison value can be a constant or an expression that uses columns from tables that are read before this table. In the following examples, MySQL can use an eq_ref join to process ref_table:
`SELECT * FROM ref_table,other_table
WHERE ref_table.key_column=other_table.column;

    SELECT * FROM ref_table,other_table
      WHERE ref_table.key_column_part1=other_table.column
      AND ref_table.key_column_part2=1;`

ref

All rows with matching index values are read from this table for each combination of rows from the previous tables. ref is used if the join uses only a leftmost prefix of the key or if the key is not a PRIMARY KEY or UNIQUE index (in other words, if the join cannot select a single row based on the key value). If the key that is used matches only a few rows, this is a good join type.

ref can be used for indexed columns that are compared using the = or <=> operator. In the following examples, MySQL can use a ref join to process ref_table:
`SELECT * FROM ref_table WHERE key_column=expr;

     SELECT * FROM ref_table,other_table
       WHERE ref_table.key_column=other_table.column;

       SELECT * FROM ref_table,other_table
   WHERE ref_table.key_column_part1=other_table.column
      AND ref_table.key_column_part2=1;`

fulltext

The join is performed using a FULLTEXT index.

ref_or_null

This join type is like ref, but with the addition that MySQL does an extra search for rows that contain NULL values. This join type optimization is used most often in resolving subqueries. In the following examples, MySQL can use a ref_or_null join to process ref_table:
SELECT * FROM ref_table
WHERE key_column=expr OR key_column IS NULL;

index_merge

This join type indicates that the Index Merge optimization is used. In this case, the key column in the output row contains a list of indexes used, and key_len contains a list of the longest key parts for the indexes used. For more information, see Section 8.2.1.3, “Index Merge Optimization”.

unique_subquery

This type replaces eq_ref for some IN subqueries of the following form:
value IN (SELECT primary_key FROM single_table WHERE
some_expr)

index_subquery

This join type is similar to unique_subquery. It replaces IN subqueries, but it works for nonunique indexes in subqueries of the following form:
value IN (SELECT key_column FROM single_table WHERE
some_expr)

range

Only rows that are in a given range are retrieved, using an index to select the rows. The key column in the output row indicates which index is used. The key_len contains the longest key part that was used. The ref column is NULL for this type.

range can be used when a key column is compared to a constant using any of the =, <>, >, >=, <, <=, IS NULL, <=>, BETWEEN, LIKE, or IN() operators:

   `SELECT * FROM tbl_name
      WHERE key_column = 10;

    SELECT * FROM tbl_name
      WHERE key_column BETWEEN 10 and 20;

    SELECT * FROM tbl_name
      WHERE key_column IN (10,20,30);

    SELECT * FROM tbl_name
      WHERE key_part1 = 10 AND key_part2 IN (10,20,30);`

index

The index join type is the same as ALL, except that the index tree is scanned. This occurs two ways:

If the index is a covering index for the queries and can be used to satisfy all data required from the table, only the index tree is scanned. In this case, the Extra column says Using index. An index-only scan usually is faster than ALL because the size of the index usually is smaller than the table data.

A full table scan is performed using reads from the index to look up data rows in index order. Uses index does not appear in the Extra column.

MySQL can use this join type when the query uses only columns that are part of a single index.

*ALL
*
A full table scan is done for each combination of rows from the previous tables. This is normally not good if the table is the first table not marked const, and usually very bad in all other cases. Normally, you can avoid ALL by adding indexes that enable row retrieval from the table based on constant values or column values from earlier tables.