editing exisiting databse -- 2

Implement the getPage() method in:

• [login to view URL]

there are no unit tests for BufferPool. The functionality you implemented will be tested in the implementation of HeapFile below. You should use the [login to view URL] method to access pages of a DbFile.

• HeapFile access method

Access methods provide a way to read or write data from disk that is arranged in a specific way. Common access methods include heap files (unsorted files of tuples) and B-trees; you will only implement a heap file access method, and we have written some of the code for you.

A HeapFile object is arranged into a set of pages, each of which consists of a fixed number of bytes for storing tuples, (defined by the constant BufferPool.DEFAULT_PAGE_SIZE), including a header. In SimpleDB, there is one HeapFile object for each table in the database. Each page in a HeapFile is arranged as a set of slots, each of which can hold one tuple (tuples for a given table in SimpleDB are all of the same size). In addition to these slots, each page has a header that consists of a bitmap with one bit per tuple slot. If the bit corresponding to a particular tuple is 1, it indicates that the tuple is valid; if it is 0, the tuple is invalid (e.g., has been deleted or was never initialized.) Pages of HeapFile objects are of type HeapPage which implements the Page interface. Pages are stored in the buffer pool but are read and written by the HeapFile class.

SimpleDB stores heap files on disk in more or less the same format they are stored in memory. Each file consists of page data arranged consecutively on disk. Each page consists of one or more bytes representing the header, followed by the page size bytes of actual page content. Each tuple requires tuple size * 8 bits for its content and 1 bit for the header. Thus, the number of tuples that can fit in a single page is:

_tuples per page_ = floor((_page size_ * 8) / (_tuple size_ * 8 + 1))

Where tuple size is the size of a tuple in the page in bytes. The idea here is that each tuple requires one additional bit of storage in the header. We compute the number of bits in a page (by multiplying page size by 8), and divide this quantity by the number of bits in a tuple (including this extra header bit) to get the number of tuples per page. The floor operation rounds down to the nearest integer number of tuples (we don't want to store partial tuples on a page!)

Once we know the number of tuples per page, the number of bytes required to store the header is simply:

headerBytes = ceiling(tupsPerPage/8)

The ceiling operation rounds up to the nearest integer number of bytes (we never store less than a full byte of header information.)

The low (least significant) bits of each byte represents the status of the slots that are earlier in the file. Hence, the lowest bit of the first byte represents whether or not the first slot in the page is in use. The second lowest bit of the first byte represents whether or not the second slot in the page is in use, and so on.

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Project ID: #29664900

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