Sometimes reading the whole index and then reading the whole table with the related row IDs will be faster than reading directly from the table. So we will perform two operations respectively and this will increase the cost. Then instead we can go to the table and read from the table directly because as you know, reading from the index also needs to read from the table in most cases. So you may think that why the optimizer performs an index full scan. So in the end, 1 million rows will be indexed somehow. Basically in any index, index entry is includes all the raw IDs of the related table. Actually, this does not mean that if we have 1 million rows, we will have 1 million entries. I mean, if our table has 1 million rows, our index will have entries for all the rows. We know that index is create index entries for all the rows. In this lecture, you will see the index full scan access path in details.
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