(mysql.info) index-merge-sort-union

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(mysql.info) index-merge-union

(mysql.info) index-merge-optimization

 
 7.2.6.3 The 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.
 
 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 rows
 and sort them before returning any rows.
 

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(mysql.info) index-merge-union

(mysql.info) index-merge-optimization