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POinscrutable response of explain and how to decide to add an index in mysql
 

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  1. POinscrutable response of explain and how to decide to add an index in mysql
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    <p>we have a very big database and to ask the question in very simple terms, I am unable to decide whether should I add an index on date field or not.</p> <p>My query is: should I add an index on date field for table A, which is very large and date (format: 2013-02-26 18:52:23). Following is my query:</p> <pre><code> SELECT As.id FROM As INNER JOIN A_items ON A_items.A_id = As.id AND A_items.type IN ('BilledItem', 'CustomerItem') WHERE (As.A_date BETWEEN '2013-01-15 18:52:23' AND '2013-01-30 18:52:23') AND A_items.category_id in ('20219') and A_items.product_id IN ('ACCDYHGYUDZNY7FZ') </code></pre> <p>now when I use explain on this it gives me following result:-</p> <pre><code> *************************** 1. row *************************** id: 1 select_type: SIMPLE table: A_items type: ref possible_keys: index_A_items_on_A_id,index_A_items_on_product_id,i_type_parent_id_item_type key: index_A_items_on_product_id key_len: 258 ref: const rows: 221122 Extra: Using where *************************** 2. row *************************** id: 1 select_type: SIMPLE table: As type: eq_ref possible_keys: PRIMARY key: PRIMARY key_len: 8 ref: database.A_items.A_id rows: 1 Extra: Using where 2 rows in set (0.00 sec) </code></pre> <p>However, when I remove A_items.product_id IN ('ACCDYHGYUDZNY7FZ') from my query and run this:-</p> <pre><code> explain SELECT As.id FROM As INNER JOIN A_items ON A_items.A_id = As.id AND A_items.type IN ('BilledItem', 'CustomerItem') WHERE (As.A_date BETWEEN '2013-01-15' AND '2013-01-30') AND A_items.category_id in ('2005') </code></pre> <p>I get :- </p> <pre><code> *************************** 1. row *************************** id: 1 select_type: SIMPLE table: As type: ALL possible_keys: PRIMARY key: NULL key_len: NULL ref: NULL rows: 15427520 &lt;--Notice this big number Extra: Using where *************************** 2. row *************************** id: 1 select_type: SIMPLE table: A_items type: ref possible_keys: index_A_items_on_A_id,i_type_parent_id_item_type key: index_A_items_on_A_id key_len: 8 ref: database.As.id rows: 1 Extra: Using where 2 rows in set (0.00 sec) </code></pre> <p>My question is why this big number of rows are not shown in the first query and how to decide whether to add index on date field or not???</p>
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    1. USSaurabh
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