Hi folks! We are back with another post. Today we will discuss in detail an important aspect of solr which is being used by a large number of enterprise applications. In most applications, there is a need to return a response to every query thrown at the solr engine within some workable contextual bounds. In order to facilitate this, we have a parser that closely resembles that of google. The intent is to make sure that for most queries, a response is returned without any errors.
To put simply, the DisMax query parser is designed to process simple phrases (without complex syntax) entered by users and to search for individual terms across several fields using different weighting (boosts) based on the significance of each field. Additional options enable users to influence the score based on rules specific to each use case (independent of user input).
An important aspect in this regard is mm (minimum match) parameter. When processing queries, Lucene/Solr recognizes three types of clauses: mandatory, prohibited, and “optional” (also known as “should” clauses). By default, all words or phrases specified in the q parameter are treated as “optional” clauses unless they are preceded by a “+” or a “-“. When dealing with these “optional” clauses, the mm parameter makes it possible to say that a certain minimum number of those clauses must match. The DisMax query parser offers great flexibility in how the minimum number can be specified.
The table below explains the various ways that mm values can be specified.
| Syntax | Example | Description |
|---|---|---|
|
Positive integer |
3 |
Defines the minimum number of clauses that must match, regardless of how many clauses there are in total. |
|
Negative integer |
-2 |
Sets the minimum number of matching clauses to the total number of optional clauses, minus this value. |
|
Percentage |
75% |
Sets the minimum number of matching clauses to this percentage of the total number of optional clauses. The number computed from the percentage is rounded down and used as the minimum. |
|
Negative percentage |
-25% |
Indicates that this percent of the total number of optional clauses can be missing. The number computed from the percentage is rounded down, before being subtracted from the total to determine the minimum number. |
|
An expression beginning with a positive integer followed by a > or < sign and another value |
3<90% |
Defines a conditional expression indicating that if the number of optional clauses is equal to (or less than) the integer, they are all required, but if it’s greater than the integer, the specification applies. In this example: if there are 1 to 3 clauses they are all required, but for 4 or more clauses only 90% are required. |
|
Multiple conditional expressions involving > or < signs |
2<-25% 9<-3 |
Defines multiple conditions, each one being valid only for numbers greater than the one before it. In the example at left, if there are 1 or 2 clauses, then both are required. If there are 3-9 clauses all but 25% are required. If there are more then 9 clauses, all but three are required. |
When specifying mm values, following aspects are to be kept in mind:
- When dealing with percentages, negative values can be used to get different behavior in edge cases. 75% and -25% mean the same thing when dealing with 4 clauses, but when dealing with 5 clauses 75% means 3 are required, but -25% means 4 are required.
- If the calculations based on the parameter arguments determine that no optional clauses are needed, the usual rules about Boolean queries still apply at search time. (That is, a Boolean query containing no required clauses must still match at least one optional clause).
- No matter what number the calculation arrives at, Solr will never use a value greater than the number of optional clauses, or a value less than 1. In other words, no matter how low or how high the calculated result, the minimum number of required matches will never be less than 1 or greater than the number of clauses.
- When searching across multiple fields that are configured with different query analyzers, the number of optional clauses may differ between the fields. In such a case, the value specified by mm applies to the maximum number of optional clauses. For example, if a query clause is treated as stopword for one of the fields, the number of optional clauses for that field will be smaller than for the other fields. A query with such a stopword clause would not return a match in that field if mm is set to 100% because the removed clause does not count as matched.
The default value of mm is 0% (all clauses optional), unless q.op is specified as “AND”, in which case mm defaults to 100% (all clauses required).
So, this is it about mm (minimum match) in solr. We will be back with another post on solr very soon.
