. ISAM index Indexed Sequential Access Method (ISAM) uses (Web design course)
. ISAM index Indexed Sequential Access Method (ISAM) uses a simple structure with a list of record numbers. ISAM indexes are used in various database engines. ISAM indexes are best used for static data as their internal list structure prohibits easy changes, making them extremely vulnerable to index overflow. . Hash table A hash table is a copy of data but rearranged into a different and more efficiently accessed order depending on a hashing algorithm. For example, a hashing algorithm takes a string and creates a number from that string. The number created by a specific string is always the same and is thus placed in a position in an index, sorted based on the hash-calculated value. Hash indexes can be highly efficient for read access, but are best avoided when subjected to any kind of data changes. Hash tables are likely to overflow worse than Bitmap indexes because there is absolutely no scope whatsoever for changes. The only way to push record changes from table to index is by regenerating the entire hash table index. . Index Organized Table An Index Organized Table (IOT) builds a table in the sorted order of an index, typically using a BTree index. IOTs can actually work fairly well in many types of databases, but you must remember that index record length is much longer than normal because index leaf blocks contain all fields in the entire record length of a table. Also, if the IOT is not read in indexed order, obviously all records in the table are read, and thus the index is ignored. Because the table is built in the structure of an index, however, not reading the table in IOT indexed order could be seriously problematic for performance. Different Ways to Build Indexes Indexes can usually be built in various different ways to accommodate however they might be used. Once again, some relational databases allow all of these options, some allow some, and some allow none. . Ascending or descending index An index can be built sorted in a normally ascending order (such as A, B, C) or in descending order (such as C, B, A). . Unique index Indexes values must be unique (can t contain duplicate values). . Non-unique index Non-unique indexes contain duplicated or repeated values in the index. It is normal to create both unique indexes and non-unique indexes. . Composite index Indexes can be built on more than a single field and are known as composite field indexes, multiple field indexes, or just plain old composite indexes. The most efficient type of index is a single field index containing an integer. . Compressed indexes Some databases allow compression of composite indexes where repeated prefix values are effectively indexed within the index, removing duplications within prefixed indexed fields. In other words, a composite index containing three fields can be accessed using a single value of the first field. . Reverse key indexes This is a really weird and unusual one. Only a very select few databases allow building of indexes such that indexed field values are stored as reverse strings. When adding gazillions of records at once to the same index in a very busy database, adding sequential index values (not reversed) adds many records all at once to the same physical space in the index. The result is what some relational databases call locking and other relational databases 68 Chapter 3
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