explain command usage

i ran the explain command on my main table in mysql.it showed like

id  select_type     table   type    possible_keys   key     key_len     ref     rows    Extra
1   SIMPLE  xyz     ALL     NULL    NULL    NULL    NULL    1722    Using where

does this affect the performance of the site? like when i so a select clause ? i have a primary key but this command says primary key 开发者_开发问答as no .

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Key - The key column indicates the key (index) that MySQL actually decided to use. The key is NULL if no index was chosen. To force MySQL to use or ignore an index listed in the possible_keys column, use FORCE INDEX, USE INDEX, or IGNORE INDEX in your query

From your query result we can see that you have no Index defined for your table because possible_keys column is also NULL.

A database index is a data structure that improves the speed of data retrieval operations on a database table at the cost of slower writes and increased storage space.

Take a look at this page to see the syntax used to create an index in MySQL:

CREATE INDEX Syntax

This page details each column of the explain plan:

Looking at the MySQL Explain Plan