each entry in the index table occupies bytesthen we need an index table of bytes bytes finding such big space consecutively is not always easy soa better scheme is to index the index table figure shows two-level multi-indexing we can continue further by having three-level indexing and so on but practicallywe use two-level indexing note that two and higher-level indexing must always be sparseotherwise multi-level indexing will lose its effectiveness in the figurethe main index table stores pointers to three inner index tables the inner index tables are sparse index tables that in turn store pointers to the records record number pointer to record record number pointer to record record number record number pointer to record records pointer to record figure multi-level indices inverted indices inverted files are commonly used in document retrieval systems for large textual databases an inverted file reorganizes the structure of an existing data file in order to provide fast access to all records having one field falling within the set limits for exampleinverted files are widely used by bibliographic databases that may store author namestitle wordsjournal namesetc when term or keyword specified in the inverted file is identifiedthe record number is given and set of records corresponding to the search criteria are created thusfor each keywordan inverted file contains an inverted list that stores list of pointers to all occurrences of that term in the main text thereforegiven keywordthe addresses of all the documents containing that keyword can easily be located there are two main variants of inverted indicesa record-level inverted index (also known as inverted file index or inverted filestores list of references to documents for each word word-level inverted index (also known as full inverted index or inverted listin addition to list of references to documents for each word also contains the positions of each word within document although this technique needs more time and spaceit offers more functionality (like phrase searches