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We now turn our attention from the way pages are stored on disk and brought into main memory to the way pages are used to store records and organized into logical collections or les Higher levels of the DBMS code treat a page as e ectively being a collection of records, ignoring the representation and storage details In fact, the concept of a collection of records is not limited to the contents of a single page; a le of records is a collection of records that may reside on several pages In this section, we consider how a collection of pages can be organized as a le We discuss how the space on a page can be organized to store a collection of records in Sections 76 and 77 Each record has a unique identi er called a record id, or rid for short As we will see in Section 76, we can identify the page containing a record by using the record s rid The basic le structure that we consider, called a heap le, stores records in random order and supports retrieval of all records or retrieval of a particular record speci ed by its rid Sometimes we want to retrieve records by specifying some condition on the elds of desired records, for example, Find all employee records with age 35 To speed up such selections, we can build auxiliary data structures that allow us to quickly nd the rids of employee records that satisfy the given selection condition Such an auxiliary structure is called an index; we introduce indexes in Section 752
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The simplest le structure is an unordered le or heap le The data in the pages of a heap le is not ordered in any way, and the only guarantee is that one can retrieve all records in the le by repeated requests for the next record Every record in the le has a unique rid, and every page in a le is of the same size Supported operations on a heap le include create and destroy les, insert a record, delete a record with a given rid, get a record with a given rid, and scan all records in the le To get or delete a record with a given rid, note that we must be able to nd the id of the page containing the record, given the id of the record We must keep track of the pages in each heap le in order to support scans, and we must keep track of pages that contain free space in order to implement insertion e ciently We discuss two alternative ways to maintain this information In each of these alternatives, pages must hold two pointers (which are page ids) for le-level bookkeeping in addition to the data
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One possibility is to maintain a heap le as a doubly linked list of pages The DBMS can remember where the rst page is located by maintaining a table containing pairs of heap f ile name, page 1 addr in a known location on disk We call the rst page of the le the header page An important task is to maintain information about empty slots created by deleting a record from the heap le This task has two distinct parts: how to keep track of free space within a page and how to keep track of pages that have some free space We consider the rst part in Section 76 The second part can be addressed by maintaining a doubly linked list of pages with free space and a doubly linked list of full pages; together, these lists contain all pages in the heap le This organization is illustrated in Figure 74; note that each pointer is really a page id
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