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In the previous chapter we used streams to move data from object models into les In one example, information was stored in a comma-delimited le as rows of information Each row had a similar composition (eg a name, address and telephone number) so that the data stored in the le was effectively a table A at- le database is similar to this with one necessary exception Since it is important that a database system can retrieve an item of information as quickly as possible, it is important that each row in the data table takes exactly the same amount of storage space as every other row Then, an operating system feature that makes it possible to jump directly to any location within a le and read the data from that point on can be used to gain direct access to any record in a table For example, Table 121 shows data that could be stored in a at- le database In the table, each row (apart from the rst, headings row) contains a record of data Each column speci es a eld, which is an atomic item of data (one that contains a single piece of information in the context of the record) During the speci cation of a table, each eld is described in terms of the type of data it may contain (number, textual characters, date, etc), and its format or maximum size (eg numeric oating point, numeric integer, maximum of 40 characters, etc) Since this type of decision xes the amount of storage space that a eld will t into, it has the advantage of making a table of data a very ef cient structure to store and to work with
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121 Object-Oriented Database Systems
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Table 121 A table of data
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Name
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Table 122 Field structure for the table in Table 121
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Name Address1 Address2 PostCode Telephone Date of Birth
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Character Character Character Character Character Date
Size in bytes
60 40 40 9 16 8
For example, assume that the elds in Table 121 are organized as shown in Table 122 Each record in the table will take exactly 173 bytes of storage space, found by adding together the byte size of each of the elds Note that the size of each character eld would be chosen by a database developer, while the date size is standard for a given database engine Given the xed record size, a routine to access it will be able to do its work very fast Record 1 will occupy bytes 1 to 173, record 2 bytes 174 to 346 and so on For example, the 31st record of data will be located from byte number (30 * 173 + 1) to byte number (31 * 173) in the le Any computer-based random-access ling system (such as the Windows FAT or NTFS ling systems) can access and return this information very quickly The limitation is of course that every record must take up exactly the same amount of space If someone s name is only 10 characters long, 50 characters of space will be wasted If they have only one line of address, the entire eld for the second line will be wasted The most obvious alternative scheme of placing a marker at the end of every eld in the table (for example, a comma) will allow data of any size to t snugly in a eld but would slow things up by forcing the ling system to count through each marker of each record to access a particular record, since no simple calculation could work out where a given record started Using a system like this, a at- le database can store and retrieve simple information quickly Provided care has been taken in deciding on the length of the variable-width elds (usually strings), the system can also be acceptably ef cient in its use of space Given the current standard of multi-gigabyte disk drives, the wasted space can be considered to be trivial