Do Bitmaps Lock Tables?
Looking at figure 2, we see in the B*tree index that every entry consists of a set of flags, a lock byte, and (in this case) two columns of data. The two columns are in fact the indexed value, and a rowid — and every row in our table has a corresponding entry of this form in the index. (If the index were a unique index, we would still see the same content in each entry, but the layout would be a little different). In the bitmap index, every entry consists of a set of flags, a lock byte, and (in this case) four columns of data. The four columns are in fact the indexed value, a pair of rowids and a stream of bits. The pair of rowids identifies a contiguous section of the table, and the stream of bits is encoded to tell us which rows in that range of rowids hold that value. Look at the size of the bit stream though — the length of the column in the example above is 3,521 bytes, or roughly 27,000 bits. Allowing about 12% overhead for check sums and so on, this single entry could cover ab
