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The slotted page organization described for variable-length records in Section 762 can also be used for xed-length records It becomes attractive if we need to move records around on a page for reasons other than keeping track of space freed by deletions A typical example is that we want to keep the records on a page sorted (according to the value in some eld)
762 Variable-Length Records
If records are of variable length, then we cannot divide the page into a xed collection of slots The problem is that when a new record is to be inserted, we have to nd an empty slot of just the right length if we use a slot that is too big, we waste space, and obviously we cannot use a slot that is smaller than the record length Therefore, when a record is inserted, we must allocate just the right amount of space for it, and when a record is deleted, we must move records to ll the hole created by the deletion,
7
in order to ensure that all the free space on the page is contiguous Thus, the ability to move records on a page becomes very important The most exible organization for variable-length records is to maintain a directory of slots for each page, with a record o set, record length pair per slot The rst component (record o set) is a pointer to the record, as shown in Figure 78; it is the o set in bytes from the start of the data area on the page to the start of the record Deletion is readily accomplished by setting the record o set to -1 Records can be moved around on the page because the rid, which is the page number and slot number (that is, position in the directory), does not change when the record is moved; only the record o set stored in the slot changes
DATA AREA rid = (i,N)
PAGE i offset of record from start of data area
rid = (i,2) Record with rid = (i,1)
Pointer to start of free space
length = 24
FREE SPACE
20 N
16 2
24 N 1
Number of entries SLOT DIRECTORY in slot directory
Page Organization for Variable-Length Records
The space available for new records must be managed carefully because the page is not preformatted into slots One way to manage free space is to maintain a pointer (that is, o set from the start of the data area on the page) that indicates the start of the free space area When a new record is too large to t into the remaining free space, we have to move records on the page to reclaim the space freed by records that have been deleted earlier The idea is to ensure that after reorganization, all records appear contiguously, followed by the available free space A subtle point to be noted is that the slot for a deleted record cannot always be removed from the slot directory, because slot numbers are used to identify records by deleting a slot, we change (decrement) the slot number of subsequent slots in the slot directory, and thereby change the rid of records pointed to by subsequent slots The
Storing Data: Disks and Files
only way to remove slots from the slot directory is to remove the last slot if the record that it points to is deleted However, when a record is inserted, the slot directory should be scanned for an element that currently does not point to any record, and this slot should be used for the new record A new slot is added to the slot directory only if all existing slots point to records If inserts are much more common than deletes (as is typically the case), the number of entries in the slot directory is likely to be very close to the actual number of records on the page This organization is also useful for xed-length records if we need to move them around frequently; for example, when we want to maintain them in some sorted order Indeed, when all records are the same length, instead of storing this common length information in the slot for each record, we can store it once in the system catalog In some special situations (eg, the internal pages of a B+ tree, which we discuss in 9), we may not care about changing the rid of a record In this case the slot directory can be compacted after every record deletion; this strategy guarantees that the number of entries in the slot directory is the same as the number of records on the page If we do not care about modifying rids, we can also sort records on a page in an e cient manner by simply moving slot entries rather than actual records, which are likely to be much larger than slot entries A simple variation on the slotted organization is to maintain only record o sets in the slots For variable-length records, the length is then stored with the record (say, in the rst bytes) This variation makes the slot directory structure for pages with xed-length records be the same as for pages with variable-length records
RECORD FORMATS *
In this section we discuss how to organize elds within a record While choosing a way to organize the elds of a record, we must take into account whether the elds of the record are of xed or variable length and consider the cost of various operations on the record, including retrieval and modi cation of elds Before discussing record formats, we note that in addition to storing individual records, information that is common to all records of a given record type (such as the number of elds and eld types) is stored in the system catalog, which can be thought of as a description of the contents of a database, maintained by the DBMS (Section 132) This avoids repeated storage of the same information with each record of a given type
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