|The PostgreSQL 9.0 Reference Manual - Volume 1A - SQL Language Reference
by The PostgreSQL Global Development Group
Paperback (6"x9"), 454 pages
RRP £14.95 ($19.95)
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10.1.1 What Is a Document?
A document is the unit of searching in a full text search system; for example, a magazine article or email message. The text search engine must be able to parse documents and store associations of lexemes (key words) with their parent document. Later, these associations are used to search for documents that contain query words.
For searches within PostgreSQL, a document is normally a textual field within a row of a database table, or possibly a combination (concatenation) of such fields, perhaps stored in several tables or obtained dynamically. In other words, a document can be constructed from different parts for indexing and it might not be stored anywhere as a whole. For example:
SELECT title || ' ' || author || ' ' || abstract || ' ' || body AS document FROM messages WHERE mid = 12; SELECT m.title || ' ' || m.author || ' ' || m.abstract || ' ' || d.body AS document FROM messages m, docs d WHERE mid = did AND mid = 12;
Note: Actually, in these example queries,
coalesceshould be used to prevent a single
NULLattribute from causing a
NULLresult for the whole document.
Another possibility is to store the documents as simple text files in the file system. In this case, the database can be used to store the full text index and to execute searches, and some unique identifier can be used to retrieve the document from the file system. However, retrieving files from outside the database requires superuser permissions or special function support, so this is usually less convenient than keeping all the data inside PostgreSQL. Also, keeping everything inside the database allows easy access to document metadata to assist in indexing and display.
For text search purposes, each document must be reduced to the
tsvector format. Searching and ranking
are performed entirely on the
of a document--the original text need only be retrieved
when the document has been selected for display to a user.
We therefore often speak of the
tsvector as being the
document, but of course it is only a compact representation of
the full document.
|ISBN 9781906966041||The PostgreSQL 9.0 Reference Manual - Volume 1A - SQL Language Reference||See the print edition|