| Postgres-XC 1.0.1 Documentation | ||||
|---|---|---|---|---|
| Prev | Fast Backward | Chapter 12. Full Text Search | Fast Forward | Next |
Full Text Searching (or just text search) provides the capability to identify natural-language documents that satisfy a query, and optionally to sort them by relevance to the query. The most common type of search is to find all documents containing given query terms and return them in order of their similarity to the query. Notions of query and similarity are very flexible and depend on the specific application. The simplest search considers query as a set of words and similarity as the frequency of query words in the document.
Textual search operators have existed in databases for years. Postgres-XC has ~, ~*, LIKE, and ILIKE operators for textual data types, but they lack many essential properties required by modern information systems:
There is no linguistic support, even for English. Regular expressions are not sufficient because they cannot easily handle derived words, e.g., satisfies and satisfy. You might miss documents that contain satisfies, although you probably would like to find them when searching for satisfy. It is possible to use OR to search for multiple derived forms, but this is tedious and error-prone (some words can have several thousand derivatives).
They provide no ordering (ranking) of search results, which makes them ineffective when thousands of matching documents are found.
They tend to be slow because there is no index support, so they must process all documents for every search.
Full text indexing allows documents to be preprocessed and an index saved for later rapid searching. Preprocessing includes:
Parsing documents into tokens. It is useful to identify various classes of tokens, e.g., numbers, words, complex words, email addresses, so that they can be processed differently. In principle token classes depend on the specific application, but for most purposes it is adequate to use a predefined set of classes. Postgres-XC uses a parser to perform this step. A standard parser is provided, and custom parsers can be created for specific needs.
Converting tokens into lexemes. A lexeme is a string, just like a token, but it has been normalized so that different forms of the same word are made alike. For example, normalization almost always includes folding upper-case letters to lower-case, and often involves removal of suffixes (such as s or es in English). This allows searches to find variant forms of the same word, without tediously entering all the possible variants. Also, this step typically eliminates stop words, which are words that are so common that they are useless for searching. (In short, then, tokens are raw fragments of the document text, while lexemes are words that are believed useful for indexing and searching.) Postgres-XC uses dictionaries to perform this step. Various standard dictionaries are provided, and custom ones can be created for specific needs.
Storing preprocessed documents optimized for searching. For example, each document can be represented as a sorted array of normalized lexemes. Along with the lexemes it is often desirable to store positional information to use for proximity ranking, so that a document that contains a more "dense" region of query words is assigned a higher rank than one with scattered query words.
Dictionaries allow fine-grained control over how tokens are normalized. With appropriate dictionaries, you can:
Define stop words that should not be indexed.
Map synonyms to a single word using Ispell.
Map phrases to a single word using a thesaurus.
Map different variations of a word to a canonical form using an Ispell dictionary.
Map different variations of a word to a canonical form using Snowball stemmer rules.
A data type tsvector is provided for storing preprocessed documents, along with a type tsquery for representing processed queries (Section 8.11). There are many functions and operators available for these data types (Section 9.13), the most important of which is the match operator @@, which we introduce in Section 12.1.2. Full text searches can be accelerated using indexes (Section 12.9).
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 Postgres-XC, 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 NULL attribute from causing a NULL result 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 Postgres-XC. 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 preprocessed tsvector format. Searching and ranking are performed entirely on the tsvector representation 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.
Full text searching in Postgres-XC is based on the match operator @@, which returns true if a tsvector (document) matches a tsquery (query). It doesn't matter which data type is written first:
SELECT 'a fat cat sat on a mat and ate a fat rat'::tsvector @@ 'cat & rat'::tsquery; ?column? ---------- t SELECT 'fat & cow'::tsquery @@ 'a fat cat sat on a mat and ate a fat rat'::tsvector; ?column? ---------- f
As the above example suggests, a tsquery is not just raw
text, any more than a tsvector is. A tsquery
contains search terms, which must be already-normalized lexemes, and
may combine multiple terms using AND, OR, and NOT operators.
(For details see Section 8.11.) There are
functions to_tsquery and plainto_tsquery
that are helpful in converting user-written text into a proper
tsquery, for example by normalizing words appearing in
the text. Similarly, to_tsvector is used to parse and
normalize a document string. So in practice a text search match would
look more like this:
SELECT to_tsvector('fat cats ate fat rats') @@ to_tsquery('fat & rat');
?column?
----------
tObserve that this match would not succeed if written as
SELECT 'fat cats ate fat rats'::tsvector @@ to_tsquery('fat & rat');
?column?
----------
fsince here no normalization of the word rats will occur. The elements of a tsvector are lexemes, which are assumed already normalized, so rats does not match rat.
The @@ operator also supports text input, allowing explicit conversion of a text string to tsvector or tsquery to be skipped in simple cases. The variants available are:
tsvector @@ tsquery tsquery @@ tsvector text @@ tsquery text @@ text
The first two of these we saw already. The form text @@ tsquery is equivalent to to_tsvector(x) @@ y. The form text @@ text is equivalent to to_tsvector(x) @@ plainto_tsquery(y).
The above are all simple text search examples. As mentioned before, full text search functionality includes the ability to do many more things: skip indexing certain words (stop words), process synonyms, and use sophisticated parsing, e.g., parse based on more than just white space. This functionality is controlled by text search configurations. Postgres-XC comes with predefined configurations for many languages, and you can easily create your own configurations. (psql's \dF command shows all available configurations.)
During installation an appropriate configuration is selected and default_text_search_config is set accordingly in postgresql.conf. If you are using the same text search configuration for the entire cluster you can use the value in postgresql.conf. To use different configurations throughout the cluster but the same configuration within any one database, use ALTER DATABASE ... SET. Otherwise, you can set default_text_search_config in each session.
Each text search function that depends on a configuration has an optional regconfig argument, so that the configuration to use can be specified explicitly. default_text_search_config is used only when this argument is omitted.
To make it easier to build custom text search configurations, a configuration is built up from simpler database objects. Postgres-XC's text search facility provides four types of configuration-related database objects:
Text search parsers break documents into tokens and classify each token (for example, as words or numbers).
Text search dictionaries convert tokens to normalized form and reject stop words.
Text search templates provide the functions underlying dictionaries. (A dictionary simply specifies a template and a set of parameters for the template.)
Text search configurations select a parser and a set of dictionaries to use to normalize the tokens produced by the parser.
Text search parsers and templates are built from low-level C functions; therefore it requires C programming ability to develop new ones, and superuser privileges to install one into a database. (There are examples of add-on parsers and templates in the contrib/ area of the Postgres-XC distribution.) Since dictionaries and configurations just parameterize and connect together some underlying parsers and templates, no special privilege is needed to create a new dictionary or configuration. Examples of creating custom dictionaries and configurations appear later in this chapter.