(mysql.info.gz) Index merge sort-union

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(mysql.info.gz) Index merge union

(mysql.info.gz) Index Merge optimization

 
 7.2.6.3 Index Merge Sort-Union Access Algorithm
 ...............................................
 
 This access algorithm is employed when the `WHERE' clause was converted
 to several range conditions combined by `OR', but for which the Index
 Merge method union algorithm is not applicable.
 
 Here are some examples:
 
      SELECT * FROM TBL_NAME WHERE KEY_COL1 < 10 OR KEY_COL2 < 20;
 
      SELECT * FROM TBL_NAME
           WHERE (KEY_COL1 > 10 OR KEY_COL2 = 20) AND NONKEY_COL=30;
 
 The difference between the sort-union algorithm and the union algorithm
 is that the sort-union algorithm must first fetch row IDs for all
 records and sort them before returning any records.
 

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(mysql.info.gz) Index merge union

(mysql.info.gz) Index Merge optimization