Is ref: NULL bad?


In the EXPLAIN below, is ref: NULL a bad thing?

mysql> explain SELECT * FROM tbl_student WHERE item_id='7846001' AND value_id IN (5, 7,9,11);
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
| 1 | SIMPLE | TBL_student | range | PRIMARY,item_id | item_id | 18 | NULL | 6 | Using where |
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Daniel WilsonCommented:
Yes, if the table in question is large.  It indicates a table scan -- no useful index found.  On a small table (e.g. 50 records), it probably doesn't matter.  But on a big one, it's a big deal.

This helps understand the results of an Explain command.

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