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<!> [[Solr3.1]] The Extended DisMax Query Parser (eDisMax) is a robust parser designed to process advanced user input directly. It is built on the original DisMaxQParserPlugin but adds many features. It searches for the query words across multiple fields with different boosts, based on the significance of each field. Additional options let you influence the score based on rules specific to each use case (independent of user input). The DisMax page has more background on the conceptual origins and behavior. This documentation has moved to the official Reference Guide: https://cwiki.apache.org/confluence/display/solr/The+Extended+DisMax+Query+Parser.
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== Overview ==
The parser takes responsibility for building a good query from the user's input using !BooleanQueries containing !DisjunctionMaxQueries across fields and boosts you specify. It also lets you provide additional boosting queries, boosting functions, and filtering queries. These options can all be specified as default parameters for the handler in your solrconfig.xml or overridden in the Solr query URL. <!> [[Solr3.6]] You can choose which fields the end user is allowed to query, and choose to disallow direct fielded searches if wanted.

== Query Syntax ==
This parser supports full Lucene !QueryParser syntax including boolean operators 'AND', 'OR', 'NOT', '+' and '-', fielded search, term boosting, fuzzy, grouping with parens, phrase search, phrase slop, numeric ranges, wildcard search and more. If there is a syntax error in the input, such as non-existing field name or unbalanced double-quotes, the input is gracefully searched as literal strings.

== Query Structure ==
For each "word" in the query string, dismax builds a !DisjunctionMaxQuery object for that word across all of the fields in the `qf` param (with the appropriate boost values and a tiebreaker value set from the `tie` param). These !DisjunctionMaxQuery objects are then put in a !BooleanQuery with the minNumberShouldMatch option set according to the `mm` param. If any other params are specified, a larger !BooleanQuery is wrapped arround the first !BooleanQuery from the `qf` options, and the other params (`bf`, `bq`, `pf`, `pf2`, `pf3`, 'ps2', 'ps3') are added as optional clauses. The only complex clause comes from from the `pf` param, which is a single !DisjuntionMaxQuery containing the whole query 'phrase' against each of the `pf` fields.

/!\ :TODO: /!\ Need more detail on the query structure generated based on input ... a picture would be nice.

== Configuration ==
Extended !DisMax is already configured in the example configuration, with the name '''edismax'''. Thus, to select the parser, use {{{defType=edismax}}} in your query, or use the local-param syntax {!edismax}

== Field aliasing / renaming ==
<!> [[Solr3.6]] You may provide virtual alias fields for users to query. This is useful either to provide a localized or easier name than what happens to be in the schema, or to provide an alias for a group of fields to support more advanced use cases such as 'what' and 'where' queries, even if there are no physical 'what' or 'where' fields.

The syntax for aliasing is {{{f.myalias.qf=realfield}}}. A user query for {{{myalias:foo}}} will be queried as {{{realfield:foo}}}.

The alias may also refer to multiple fields, with boost factors, by listing the field names with a space between them, and the optional boost factor immediately following the field name and the caret ('{{{^}}}') operator. Let's imagine you have a schema with fields {{{name, namealias, address, city, state}}}, and you want to provide a 'who' and 'where' search. You could then configure aliases like this: {{{&f.who.qf=name^5.0+namealias^2.0&f.where.qf=address^1.0+city^10.0+state}}}. Any user query for {{{who:foo}}} would expand to a DisMax query across fields name and namealias. If you further want to hide the real field names, you can combine this with "User Fields" feature, and say {{{&uf=who,where}}} to only allow fielded search for those two aliases.

== Parameters ==
The following parameters are supported, either as regular request params, or as local params

=== q.alt ===
If specified, this query will be used (and parsed by default using standard query parsing syntax) when the main query string is not specified or blank. This comes in handy when you need something like a match-all-docs query (don't forget &rows=0 for that one!) in order to get collection-wise faceting counts.

=== qf (Query Fields) ===
List of fields and the "boosts" to associate with each of them when building !DisjunctionMaxQueries from the user's query. The format supported is {{{fieldOne^2.3 fieldTwo fieldThree^0.4}}}, which indicates that fieldOne has a boost of 2.3, fieldTwo has the default boost, and fieldThree has a boost of 0.4 ... this indicates that matches in fieldOne are much more significant than matches in fieldTwo, which are more significant than matches in fieldThree.

=== mm (Minimum 'Should' Match) ===
When dealing with queries there are 3 types of "clauses" that Lucene knows about: mandatory, prohibited, and 'optional' (aka: "SHOULD") By default all words or phrases specified in the "q" param are treated as "optional" clauses unless they are preceeded by a "+" or a "-". When dealing with these "optional" clauses, the "mm" option makes it possible to say that a certain minimum number of those clauses must match (mm). Specifying this minimum number can be done in complex ways, equating to ideas like...

 * At least 2 of the optional clauses must match, regardless of how many clauses there are: "{{{2}}}"
 * At least 75% of the optional clauses must match, rounded down: "{{{75%}}}"
 * If there are less than 3 optional clauses, they all must match; if there are 3 or more, then 75% must match, rounded up: "{{{2<-25%}}}"
 * If there are less than 3 optional clauses, they all must match; for 3 to 5 clauses, one less than the number of clauses must match, for 6 or more clauses, 80% must match, rounded down: "{{{2<-1 5<80%}}}"

Full details on the variety of complex expressions supported are explained in detail [[http://lucene.apache.org/solr/api/org/apache/solr/util/doc-files/min-should-match.html|here]].

<!> From [[Solr4.0]] The default value of mm is dictated by the q.op param (q.op=AND => mm=100%; q.op=OR => mm=0%). Keep in mind the default operator is effected by your schema.xml <solrQueryParser defaultOperator="xxx"/> entry. In older versions of Solr the default value of 'mm' is 100% (all clauses must match).

=== qs (Query Phrase Slop) ===
Amount of slop on phrase queries explicitly included in the user's query string (in qf fields; affects matching). <!> [[Solr1.3]]

=== pf (Phrase Fields) ===
Once the list of matching documents has been identified using the "fq" and "qf" params, the "pf" param can be used to "boost" the score of documents in cases where all of the terms in the "q" param appear in close proximity.

The format is the same as the "qf" param: a list of fields and "boosts" to associate with each of them when making phrase queries out of the entire "q" param.

<!> [[Solr4.0]] You can also specify an optional slop factor directly in "pf" with the syntax {{{field~slop}}}. To specify both a slop and a boost, use {{{field~slop^boost}}}. Example: {{{title~2^10.0}}} will use the title field with a phrase slop of 2 and a boost of 10.0. A phrase slop specified here overrides the default specified in "ps". See SOLR-2058.

=== ps (Phrase Slop) ===
Default amount of slop on phrase queries built with "pf", "pf2" and/or "pf3" fields (affects boosting).

=== pf2 (Phrase bigram fields) ===
As with 'pf' but chops the input into bi-grams, e.g. "the brown fox jumped" is queried as "the brown" "brown fox" "fox jumped"

=== ps2 (Phrase bigram slop) ===

<!> [[Solr4.0]] As with 'ps' but sets default slop factor for 'pf2'. If not specified, 'ps' will be used.

=== pf3 (Phrase trigram fields) ===
As with 'pf' but chops the input into tri-grams, e.g. "the brown fox jumped" is queried as "the brown fox" "brown fox jumped"

=== ps3 (Phrase trigram slop) ===

<!> [[Solr4.0]] As with 'ps' but sets default slop factor for 'pf3'. If not specified, 'ps' will be used.

=== tie (Tie breaker) ===
Float value to use as tiebreaker in !DisjunctionMaxQueries (should be something much less than 1)

When a term from the users input is tested against multiple fields, more than one field may match and each field will generate a different score based on how common that word is in that field (for each document relative to all other documents). By default the score from the field with the maximum score is used. If two documents both have a matching score, the tie parameter has the effect of breaking the tie.
When a tie parameter is specified the scores from other matching fields are added to the score of the maximum scoring field:

(score of matching clause with the highest score) + ( (tie paramenter) * (scores of any other matching clauses) )

The "tie" param let's you configure how much the final score of the query will be influenced by the scores of the lower scoring fields compared to the highest scoring field.

A value of "0.0" makes the query a pure "disjunction max query" -- only the maximum scoring sub query contributes to the final score. A value of "1.0" makes the query a pure "disjunction sum query" where it doesn't matter what the maximum scoring sub query is, the final score is the sum of the sub scores. Typically a low value (ie: 0.1) is useful.

=== bq (Boost Query) ===
A raw query string (in the SolrQuerySyntax) that will be included with the user's query to influence the score. If this is a !BooleanQuery with a default boost (1.0f) then the individual clauses will be added directly to the main query. Otherwise, the query will be included as is.

/!\ :TODO: /!\ That latter part is deprecated behavior but still works. It can be problematic so avoid it.

=== bf (Boost Function, additive) ===
[[FunctionQuery|Functions]] (with optional boosts) that will be included in the user's query to influence the score. Any function supported natively by Solr can be used, along with a boost value, e.g.: recip(rord(myfield),1,2,3)^1.5

Specifying functions with the "bf" param is just shorthand for using the {{{_val_:"...function..."}}} syntax in a "bq" param.

For example, if you want to show more recent documents first, use recip(ms(NOW,mydatefield),3.16e-11,1,1). See FunctionQuery for more functions.

The bf parameter may be specified multiple times.

=== boost (Boost Function, multiplicative) ===
As for 'bf' but multiplies the boost into the score.

=== uf (User Fields) ===
Specifies which schema fields the end user shall be allowed to query for explicitly. This parameter supports wildcards.

The default is to allow all fields, equivalent to {{{&uf=*}}}. To allow only title field, use {{{&uf=title}}}, to allow title and all fields ending with _s, use {{{&uf=title *_s}}}. To allow all fields except title, use {{{&uf=* -title}}}. To disallow all fielded searches, use {{{&uf=-*}}}.

The uf parameter was introduced in <!> [[Solr3.6]]

=== lowercaseOperators ===
This param controls whether to try to interpret lowercase words as boolean operators such as "and" and "or". Set {{{&lowercaseOperators=true}}} to allow this. Default is "true".

Please see [[https://issues.apache.org/jira/browse/SOLR-3580|SOLR-3580]] for patches to enabling lowercase "not" operator support.

== Examples ==
/!\ :TODO: /!\ cleanup and expand examples

Search across multiple fields, specifying (via boosts) how important each field is relative each other

You can boost results that have a field that matches a specific value...

Using the "mm" param, 1 and 2 word queries require that all of the optional clauses match, but for queries with three or more clauses one missing clause is allowed...


== References ==
* [[https://issues.apache.org/jira/browse/SOLR-2368|SOLR-2368]] tracks improvements to eDisMax
The reference guide is also available in PDF format for each specific Solr release, see https://archive.apache.org/dist/lucene/solr/ref-guide/

This documentation has moved to the official Reference Guide: https://cwiki.apache.org/confluence/display/solr/The+Extended+DisMax+Query+Parser.

The reference guide is also available in PDF format for each specific Solr release, see https://archive.apache.org/dist/lucene/solr/ref-guide/

ExtendedDisMax (last edited 2016-03-01 22:22:57 by JanHoydahl)