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.
