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When the multicolumn index is accessed, the main portion of the index (the index on the first column) is accessed first. In a three column index we can see that the main index stores pointers to both the original table and the reference table on make, which in turn has pointers to the reference table on model. Adding a third column to the index causes the index to look like this: Now the index has pointers to a secondary reference table that is sorted by make. Adding a second column to the index looks like this: The index points back to the table and is sorted by year. Take for example this table:Ī traditional index on this table would look like this: Multicolumn indexes are structured to have a hierarchical structure. This is due to the structure that multicolumn indexes possess. When creating a multicolumn index, the column order is very important. Multicolumn indexes are indexes that store data on up to 32 columns.
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