java aztec barcode library Fixed-Length Records in Software

Creation Data Matrix in Software Fixed-Length Records

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Record formats in commercial systems: In IBM DB2, xed length elds are at xed o sets from the beginning of the record Variable length elds have o set and length in the xed o set part of the record, and the elds themselves follow the xed length part of the record Informix, Microsoft SQL Server, and Sybase ASE use the same organization with minor variations In Oracle 8, records are structured as if all elds are potentially variable length; a record is a sequence of length data pairs, with a special length value used to denote a null value
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In a xed-length record, each eld has a xed length (that is, the value in this eld is of the same length in all records), and the number of elds is also xed The elds of such a record can be stored consecutively, and, given the address of the record, the address of a particular eld can be calculated using information about the lengths of preceding elds, which is available in the system catalog This record organization is illustrated in Figure 79
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F1 L1 F2 L2 F3 L3 F4 L4
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Fi = Field i Li = Length of field i
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Base address (B)
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Address = B+L1+L2
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Organization of Records with Fixed-Length Fields
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In the relational model, every record in a relation contains the same number of elds If the number of elds is xed, a record is of variable length only because some of its elds are of variable length One possible organization is to store elds consecutively, separated by delimiters (which are special characters that do not appear in the data itself) This organization requires a scan of the record in order to locate a desired eld An alternative is to reserve some space at the beginning of a record for use as an array of integer o sets the ith integer in this array is the starting address of the ith eld value relative to the start of the record Note that we also store an o set to the end of the record; this o set is needed to recognize where the last eld ends Both alternatives are illustrated in Figure 710
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F1 F2 F3 F4
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Fi = Field i
Fields delimited by special symbol $
Array of field offsets
Figure 710 Alternative Record Organizations for Variable-Length Fields
The second approach is typically superior For the overhead of the o set array, we get direct access to any eld We also get a clean way to deal with null values A null value is a special value used to denote that the value for a eld is unavailable or inapplicable If a eld contains a null value, the pointer to the end of the eld is set to be the same as the pointer to the beginning of the eld That is, no space is used for representing the null value, and a comparison of the pointers to the beginning and the end of the eld is used to determine that the value in the eld is null Variable-length record formats can obviously be used to store xed-length records as well; sometimes, the extra overhead is justi ed by the added exibility, because issues such as supporting null values and adding elds to a record type arise with xed-length records as well Having variable-length elds in a record can raise some subtle issues, especially when a record is modi ed Modifying a eld may cause it to grow, which requires us to shift all subsequent elds to make space for the modi cation in all three record formats presented above A record that is modi ed may no longer t into the space remaining on its page If so, it may have to be moved to another page If rids, which are used to point to a record, include the page number (see Section 76), moving a record to another page causes a problem We may have to leave a forwarding address on this page identifying the new location of the record And to ensure that space is always available for this forwarding address, we would have to allocate some minimum space for each record, regardless of its length A record may grow so large that it no longer ts on any one page We have to deal with this condition by breaking a record into smaller records The smaller
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