Abstract:
A computer-implemented method for delivering content is provided, including specifying a rule having a trigger and defining a content presentation action, and receiving a search query from a user. The method further includes generating a result set in response to the search query, identifying at least one characteristic of the result set, and comparing the at least one characteristic to the trigger. If the at least one characteristic satisfies the trigger, the content is presented as specified by the content presentation action that is defined by the rule. Other embodiments are also described.

Description:
RELATED APPLICATION 
     The present application is a continuation of U.S. patent application Ser. No. 11/497,219 now U.S. Pat. No. 7,493,317, filed 31 Jul. 2006, which claims priority to U.S. provisional patent application Ser. No. 60/729,111 filed 20 Oct. 2006, each of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates generally to online search engines, and specifically to techniques for rules-based online content presentation. 
     2. Related Art 
     Many search engines provide content manipulation tools that modify a result set in order to merchandise particular items. Most basically, search engines present results ranked by their relevance to the user&#39;s query, determined by information-retrieval considerations and search algorithms. In addition, some search engines modify the ranking of the results based on additional considerations, such as business considerations. For example, in response to the query “bicycle,” a search engine may choose to promote a certain brand within the search results, and display some of the models of this brand among the top ten results. 
     SUMMARY 
     Embodiments of the present invention enable rules for manipulating content to be triggered by search results that are returned in response to a query posed by a user. In some of these embodiments, a search and navigation system permits a manager of the content to identify characteristics that may occur in the query results, and to specify rules to be applied by the system when the characteristics appear in the results. Typically, the rules define actions to be taken by the system, such as modifying the order of the result set, adding items to the set that do not necessarily satisfy the query, or presenting content to the user that is outside the scope of the query. This approach expands the range of tools available to the manager for inferring types of content that could be of interest to the user and thus bringing the content to the user&#39;s attention. 
     There is therefore provided, in accordance with an embodiment of the present invention, a computer-implemented method for delivering content, including: specifying a rule having a trigger and defining a content presentation action; receiving a search query from a user; generating a result set in response to the search query; identifying at least one characteristic of the result set; comparing the at least one characteristic to the trigger; and if the at least one characteristic satisfies the trigger, presenting the content as specified by the content presentation action that is defined by the rule. 
     For some queries, the at least one characteristic is not included in the search query. For some applications, the search query, the at least one characteristic, and the trigger include respective attribute-value pairs. For some applications, the at least one characteristic includes at least one ordered characteristic. 
     In an embodiment of the present invention, identifying the at least one characteristic includes determining that the at least one characteristic dominates the result set. For example, determining that the at least one characteristic dominates the result set may include determining that the at least one characteristic characterizes at least a threshold percentage of items in the result set. Alternatively, determining that the at least one characteristic dominates the result set may include determining that the at least one characteristic characterizes at least a threshold number of items in the result set. 
     In an embodiment of the present invention, identifying includes assessing, for each of a plurality of characteristics that include the at least one characteristic, a number of items in the result set that are characterized by the characteristic. For some applications, the plurality of characteristics includes only characteristics included in the trigger. 
     In an embodiment of the present invention, the trigger specifies a threshold number of items, the at least one characteristic of the result set specifies a number of items in the result set, and presenting the content includes presenting the content as specified by the content presentation action that is defined by the rule responsively to a comparison between the number of items and the threshold number of items. Furthermore, presenting the content may include presenting the content as specified by the content presentation action that is defined by the rule both (a) responsively to the comparison, and (b) upon finding that the query contains a set of one or more characteristics. 
     For some applications, the trigger specifies a set of one or more items, and comparing the at least one characteristic to the trigger includes determining whether or not the result set contains the items in the specified set. 
     For some applications, identifying the at least one characteristic of the result set includes identifying a first characteristic of the result set, and a second characteristic of the result set that refines the first characteristic, and identifying the second characteristic, but not the first characteristic, as the at least one characteristic of the result set. 
     There is further provided, in accordance with an embodiment of the present invention, a computer-implemented method for delivering content, including: specifying a rule, which has a trigger and a threshold and defines a content presentation action; receiving a search query from a user; generating a query result set by searching a corpus of data for items that satisfy the search query; generating a trigger result set by searching the corpus of data for items that satisfy the trigger; determining a level of intersection between the query result set and the trigger result set; and if the level of intersection is greater than the threshold, presenting the content as specified by the content presentation action defined by the rule. For some applications, the search query and the trigger each include at least one attribute-value pair. 
     In an embodiment of the present invention, determining the level of intersection includes setting the level of intersection equal to a quotient of (i) an assessment of a number of items in a set formed by an intersection of the query and trigger result sets divided by (ii) a divisor, and the divisor is selected from the group consisting of: an assessment of a number of items in the query result set, an assessment of a number of items in the trigger result set, and a sum of the assessment of the number of items in the query result set and the assessment of the number of items in the trigger result set. 
     Alternatively, determining the level of intersection includes setting the level of intersection equal to an assessment of a number of items in a set formed by the intersection of the query and trigger result sets. 
     For some applications, generating the trigger result set includes making a record of the trigger result set, and determining the level of intersection includes determining the level of intersection between the query result set and the record of the trigger result set. 
     There is still further provided, in accordance with an embodiment of the present invention, apparatus for delivering content, including: a search engine, which is configured to receive a search query from a user, and to generate a result set in response to the search query; and a result processor, which is configured to receive a specification of a rule having a trigger and defining a content presentation action, to identify at least one characteristic of the result set, to compare the at least one characteristic to the trigger, and, if the at least one characteristic satisfies the trigger, to present the content as specified by the content presentation action that is defined by the rule. 
     There is additionally provided, in accordance with an embodiment of the present invention, apparatus for delivering content, including: a result processor, which is configured to receive a specification of a rule, which has a trigger and a threshold and defines a content presentation action; and a search engine, which is configured to receive a search query from a user, to generate a query result set by searching a corpus of data for the items that satisfy the search query, and to generate a trigger result set by searching the corpus of data for the items that satisfy the trigger, wherein the result processor is configured to determine a level of intersection between the query result set and the trigger result set, and, if the level of intersection is greater than the threshold, to present the content as specified by the content presentation action defined by the rule. 
     There is yet additionally provided, in accordance with an embodiment of the present invention, a computer software product for delivering content, the product including a computer-readable medium in which program instructions are stored, which instructions, when read by a computer, cause the computer to receive a specification of a rule having a trigger and defining a content presentation action, to receive a search query from a user, to generate a result set in response to the search query, to identify at least one characteristic of the result set, to compare the at least one characteristic to the trigger, and, if the at least one characteristic satisfies the trigger, to present the content as specified by the content presentation action that is defined by the rule. 
     There is also provided, in accordance with an embodiment of the present invention, a computer software product for delivering content, the product including a computer-readable medium in which program instructions are stored, which instructions, when read by a computer, cause the computer to receive a specification of a rule, which has a trigger and a threshold and defines a content presentation action, to receive a search query from a user, to generate a query result set by searching a corpus of data for items that satisfy the search query, to generate a trigger result set by searching the corpus of data for items that satisfy the trigger, to determine a level of intersection between the query result set and the trigger result set, and, if the level of intersection is greater than the threshold, to present the content as specified by the content presentation action defined by the rule. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be more fully understood from the following detailed description of embodiments thereof, taken together with the drawings, in which: 
         FIG. 1  is a schematic illustration of a search and navigation system  10 , in accordance with an embodiment of the present invention; 
         FIG. 2  is a flow chart that schematically illustrates a method for performing result-characteristics-based triggering, in accordance with an embodiment of the present invention; and 
         FIG. 3  is a flow chart that schematically illustrates a method for performing result-set-based triggering, in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Overview 
     Embodiments of the present invention enable rules for manipulating content to be triggered by search results that are returned in response to a query posed by a user. In some of these embodiments, a search and navigation system permits a manager of the content to identify characteristics that may occur in the query results, and to specify rules to be applied by the system when the characteristics appear in the results. Typically, the rules define actions to be taken by the system, such as modifying the order of the result set, adding items to the set that do not necessarily satisfy the query, or presenting content to the user that is outside the scope of the query. This approach expands the range of tools available to the manager for inferring types of content that could be of interest to the user and thus bringing the content to the user&#39;s attention. For some applications, such content manipulation is performed in order to merchandise items in the result set, or other items not found in the result set. 
     In some embodiments of the present invention, the system identifies characteristics of the result set that dominate the result set. Characteristics are considered dominant if they characterize at least a threshold percentage of the items in the result set, or at least a threshold number of the items in the result set. 
     These results-based content manipulation techniques enable a manager of the content to perform desired content manipulation actions based on characteristics of items for which a user is searching, even if these characteristics do not appear in the user&#39;s query. The manager does not need to define specific triggers for every possible query characteristic that may be associated with a desired content manipulation action. 
     The following example illustrates some of the differences between these techniques of the present invention and conventional query-based content presentation techniques. Assume a trigger of a content presentation rule is defined as the attribute-value pair brand: Sony®. Using conventional query-based techniques, at least the following attribute-value-based queries would produce a match with the trigger: (a) brand: Sony, and (b) product: stereo AND brand: Sony, and the following queries would fail to produce a match: (c) brand: Toshiba®, and (d) product: stereo. 
     Using these techniques of the present invention, the above-mentioned queries (a) and (b) would also produce a match, because all of the items in the result set are characterized by brand: Sony, and query (c) would still fail to produce a match, because none of the results for brand: Toshiba are characterized by brand: Sony. However, in contrast to conventional query-based content presentation techniques, query (d) (product: stereo) would also produce a match if the items in the result set are dominated by Sony, e.g., if at least 40% of the items in the result set are characterized by brand: Sony, assuming a threshold setting of 40%. These techniques thus enable the manager to determine that the user has a potential interest in Sony, which interest was not detectable in the information provided by the query alone. The hypothetical connection in this example between stereo products and the particular brand is entirely data-driven. 
     In some embodiments of the present invention, the system generates (a) a query result set in response to a user query provided as input to the search engine, and (b) a trigger result set in response to a trigger provided as a query input to the search engine. The system performs a content manipulation action upon finding a threshold level of intersection between the query result set and the trigger result set. 
     System Description 
       FIG. 1  is a schematic illustration of a search and navigation system  10 , in accordance with an embodiment of the present invention. System  10  comprises a search engine  20 , a result processor  22 , an interface, such as a web server  24 , and a memory  26 . System  10  provides searching and navigation of items stored in memory  26 . Typically, system  10  comprises one or more standard computer servers with appropriate memory, communication interfaces and software for carrying out the functions prescribed by the present invention. This software may be downloaded to the system in electronic form over a network, for example, or it may alternatively be supplied on tangible media, such as CD-ROM. 
     A user, such as a customer, uses a workstation  30 , such as a personal computer, to remotely access system  10  via a wide-area network (WAN)  32 , such as the Internet. Typically, a web browser  34  of workstation  30  communicates with web server  24 . The web browser facilitates entry of search queries, and displays search results. 
     Result processor  22  implements one or more content presentation rules, which include respective triggers and content presentation actions to be performed upon satisfaction of the triggers. Such content presentation actions may include, for example, modifying the order of the result set, adding items to the set that do not necessarily satisfy the query, or presenting content to the user that is outside the scope of the query. System  10  presents content associated with the result set, as specified by the content presentation actions corresponding to the satisfied triggers of the rules. 
     Each of the triggers comprises a set of one or more characteristics that characterize items in memory  26 . The characteristics are typically expressed as one or more attribute-value pairs (notated herein as a:v), one or more free text searches, or a combination of attribute-value pairs and free text searches. Memory  26 , or another element of system  10 , contains at least one index that associates characteristics with items in the memory. Memory  26  may be part of system  10  (such as part of one or more of search engine  20  and/or result processor  22 ), or may be distributed in other physical locations. 
     For applications in which the characteristics are expressed as attribute-value pairs, each trigger can be expressed as follows:
 
t=a 1 :v 1  operator a 2 :v 2 , . . . , operator a n :v n   (Equation 1)
 
wherein a i  is an attribute and v 1  is a value of a i , and the operators are typically Boolean operators, such as AND, OR, NOT, and parentheses. The same attribute may appear more than once in a given trigger, with each occurrence having a different value. For some applications, additional attribute-value pairs are provided to control the results of a query, which pairs are not directly related to attributes of items in memory  26 . For example, such attribute-value pairs may indicate a desired sort order, a desired page number, or a desired currency for displaying prices. Furthermore, attribute-value pairs may include additional information, such as display information. For example, the attribute-value pair brand: 14: ‘Sony’ may indicate a that the attribute “brand” has as its value a brand code of 14, which is to be displayed as “Sony.”
 
     For some applications, free text searches are expressed as attribute-value pairs, in which the attribute is a generic indicator of a free text search, and the value is the free text. Alternatively, the attribute name is not an actual specific attribute, but instead indicates the kind of free search to be carried out, e.g., a search by author, or a search by title. For example, the author search may actually refer to author first name and author last name fields. 
     Content presentation rules are expressed most simply as (trigger, action). (For some applications, such rules have more complex structures, such as the specification of a plurality of actions, or the use of a Boolean expression including a plurality of triggers.) Result processor  22  evaluates the trigger by determining whether appropriate attribute-value pairs of the trigger, as specified by the trigger&#39;s Boolean operators, match at least one attribute-value pair of an input, such as a query, or a characterization of a result set, as described hereinbelow. If so, the trigger is true, the rule is said to fire, and the action is performed; if not, the trigger is false, the rule does not fire, and the action is not performed. (For triggers that simply include a list of one more attribute-value pairs related to one another by an implied AND, the result processor evaluates the trigger by determining whether each attribute-value pair of the trigger matches at least one attribute-value pair of the input.) 
     For hierarchical attributes, a trigger characteristic a:v′ is considered to match an input characteristic a:v′ if V′ refines v, i.e., the set of items characterized by a:v′ is a subset of the set of items characterized by a:v. For some applications, a rule is triggered if the value v′ of an input characteristic a:v′ refines a value v of a trigger characteristic a:v. 
     For ordered attributes, which have a fully ordered domain of values, such as price or date, a trigger characteristic a[n 1 ,n 2 ] is considered to match an input characteristic a[n 1 ′,n 2 ′] if [n 1 ′,n 2 ′] is a sub-range of [n 1 ,n 2 ], i.e., n 1 &lt;=n 1 ′ and n 2 ′&lt;=n 2 . For some applications, for free text attributes, a trigger characteristic a:v is considered to match an input characteristic a:v′ if v is identical to v′, or if there is a linguistic association between v and v′, as is known in the search engine art. Alternatively, result-set-based triggering can be used for a similar purpose, without resorting to defining satisfaction for individual trigger components. 
     Memory  26  contains one or more data structures including items and information describing the items, such as attributes of the items. For example, the data structures may include one or more indices or tables having entries, each of which includes an attribute-value pair and an associated item. The tables may be stored in an multi-dimensional array, a linked list, a link list of arrays, or another appropriate data structure known in the art. Alternatively or additionally, memory  26  may comprise one or more databases, such as relational databases. Memory  26  generally further includes data structures that define relationships among at least some of the attribute-value pairs, such as hierarchical relationships. 
     Reference is made to  FIG. 2 , which is a flow chart that schematically illustrates a method for performing result-characteristics-based triggering, in accordance with an embodiment of the present invention. Result processor  22  uses this method to evaluate triggers  48  against input characteristics that include (a) a query  50 , and (b) inferred characteristics of the result set for the query. Alternatively, the input characteristics include only the inferred characteristics of the result set, and not query  50 . 
     The method receives as input query  50 , which, as described hereinabove, is typically expressed as attribute-value pairs, attribute-range-value pairs, and/or attribute-free text pairs. At a search step  52 , search engine  20  ( FIG. 1 ) searches query  50  against memory  26 , and outputs a result set  54  containing zero or more items from memory  26 . 
     Result processor  22  begins the characterization of the result set by assessing, for each attribute-value pair, the number of items in result set  54  that are characterized by the attribute-value pair, at a result set characterization step  56 . Such assessing typically comprises counting, either exactly or approximately, the number of items. For some applications, the result processor performs such assessing for all attribute-value pairs that have been indexed in memory  26 , while for other applications, the result processor performs such assessing only for attribute-pairs that are included in at least one of triggers  48 . In order to facilitate such assessing, memory  26  typically includes an index, such as of (items, attribute names) mapped to attribute values. It is noted that often more than one attribute-value pair per attribute is assessed. The result of the characterization is a set  58  of characteristics (e.g., attribute-value pairs) and corresponding assessments of items from result set  54  that map to the characteristics. 
     For hierarchical attributes, such as those that define categories, result processor  22  optionally characterizes each item by the most specific attribute value that is applicable to the item. In other words, if an item is characterized by both attribute-value pair a:v and a:v′, wherein v′ refines v, result processor  22  characterizes the item by a:v′ rather than a:v, assuming that v′ is above the applicable threshold. 
     At a threshold comparison step  60 , result processor  22  compares the assessments of each attribute of set  58  to one or more thresholds  62 . Thresholds  62  are specified globally, per attribute, per trigger  48 , or per each attribute of each trigger  48 . For some applications, the thresholds are expressed as item assessments, such as exact or approximate counts. For some applications, the thresholds are expressed as a percentage of the total number of items in result set  54  (or, equivalently, a range from 0 to 1, or another ratio). For example, the percentage may be at least 40%, such as at least 50%, 60%, 70%, or 80%. Alternatively or additionally, all or a portion of the thresholds are expressed as absolute numbers. Result processor  22  outputs a set  64  of those characteristics of set  58  that exceed the threshold(s) (as mentioned above, when trigger-specific thresholds are provided, this comparison is performed using the trigger-specific thresholds). The result processor thus identifies one or more characteristics that dominate result set  54 . For some applications, the result processor is configured to identify only a subset of the characteristics found, such as a single dominant characteristic (such as the most dominant characteristic), even in result set characterizations in which multiple characteristics are found. Alternatively, for some applications, the result processor is configured to identify only a single dominant characteristic per attribute. 
     At a trigger evaluation step  66 , result processor  22  evaluates each of triggers  48  against (a) set  64  of the characteristics that exceed the threshold(s), and (b) query  50 . Alternatively, the result processor evaluates trigger  48  only against set  64 . Such evaluation is performed as described hereinabove with reference to  FIG. 1 . Based on this evaluation, the result processor outputs a set  70  of satisfied triggers. At a rule application step  72 , the system performs appropriate content presentation actions for the satisfied triggers, based on the content presentation rules, as described hereinabove, and presents content to the user as specified by the rules, at a content presentation step  74 . Typically, such content relates to at least a portion of result set  54 , and includes additional content as specified by the rules. 
     For some applications, a trigger is satisfied upon finding that result set  54  is not dominated by the one or more characteristics of the trigger. For some applications, a trigger is satisfied upon finding that result set  54  contains exactly a certain number of items characterized by the one or more characteristics of the trigger, or less than a threshold number of items characterized by the one or more characteristics of the trigger. 
     In an embodiment of the present invention, the result-characteristics-based triggering described hereinabove with reference to  FIG. 2  supports ordered attributes. As mentioned above with reference to  FIG. 1 , an ordered-attribute trigger takes the form of a[n 1 ,n 2 ], wherein n 1  and n 2  are the lower and upper bounds, respectively, of the range of values of the attribute. Ordered attributes commonly include price and date. The attribute-value pair for any single given item in memory  26  is expressed as a:n, i.e., the item typically has only a single value n for the attribute a, e.g., a price of $10. 
     For supporting ordered attributes, the method of  FIG. 2  includes, at an ordered attribute characterization step  80 , further characterizing those characteristics of set  58  which are ordered (as mentioned above, set  58  is produced at result set characterization step  56 ). Such further characterization results in a set  82  of range characteristics and corresponding counts of items from result set  54  that map to the range. In other words, set  82  includes one or more attribute-value ranges a[n 1 ,n 2 ] and corresponding counts. At threshold comparison step  60 , result processor  22  compares the counts of each attribute-value range of set  82  to thresholds  62 . Attribute-value ranges having a count that exceeds an appropriate threshold (either in absolute terms, or as a percentage, as described hereinabove with reference to step  60 ) are included in output set  64 . 
     In accordance with a first technique for characterizing ordered attributes at step  80 , result processor  22  receives as input a set of desired ordering thresholds  84 . Each of ordering thresholds  84  are typically expressed as a percentage of the total items in result set  54 . Optionally, one or more of thresholds  62  serve as one or more of ordering thresholds  84 . For each ordered attribute a:n in set  58  and each ordering threshold  84 , result processor  22  produces one or more attribute-value ranges a: [n 1 ,n 2 ] by (a) setting n 1  equal to n, and (b) setting n 2  equal to the lowest possible value of attribute a greater than n 1  such that the quotient of (i) the sum of counts for a:n in set  58  over all n in [n 1 ,n 2 ] divided by (ii) the total number of items in result set  54 , is greater than the ordering threshold. Alternatively n 2  is set to n, or n 1  and n 2  are set such that n falls between n 1  and n 2 , such as at the midpoint of n 1  and n 2 . Result processor  22  outputs as set  82  each of the thus determined attribute-value ranges a: [n 1 ,n 2 ] and its corresponding count of items in result set  54 . It is noted that this technique for characterizing ordered attributes does not require knowledge of triggers  48 , and thus may be used for purposes other than triggering. 
     In accordance with a second technique for characterizing ordered attributes at step  80 , for each attribute-value range a: [n 1 ,n 2 ] included in at least one trigger  48 , result processor  22  sums the counts for a:n in set  58  over all n in [n 1 ,n 2 ], and outputs as set  82  each of the attribute value ranges and its corresponding summed count of items in result set  54 . 
     In accordance with a third technique for characterizing ordered attributes at step  80 , result processor  22  receives as input a set of desired ordering thresholds  84 , which are expressed as a set of ranges, or parameters for deriving a set of ranges from the ranges provided by triggers  48 . For example, the endpoints of the ranges may be multiples of $10, such that the ranges are [$0,$10], [$10,$20], [$20,$30], . . . . Result processor  22  sums the counts for a:n in set  58  over all n in each of the ranges, and outputs as set  82  each of the attribute value ranges and its corresponding summed count of items in result set  54 . 
     In accordance with an alternative method for supporting ordered attributes, the method of  FIG. 2  does not include step  80 . Instead, ordered attributes-value pairs included in set  58  are retained as single-value attribute-value pairs. At a combined threshold comparison step  60  and trigger evaluation step  66 , an attribute-range trigger is satisfied if the sum of the item counts falling within the range defined by the trigger exceeds a threshold value. 
     (It is noted that these attribute-range techniques produce precise results only if none of the unique items in result set  54  includes more than one value for a given attribute. Unique items could include more than value, for example, if a given item has two different SKUs with different respective prices. If any items include more than one value, summing the counts for such items may count a given unique item more than once. For some applications, result processor  22  is configured to ignore such overlaps, in which case the results of the method of  FIG. 2  are approximate. Additional methods for counting can also provide approximate counts utilizing less processing power, such as random sampling. Alternatively, to arrive at precise results, the result processor determines unions of sets of items mapping to each characteristic, such that overlapping items are not counted more than once.) 
     In the embodiments described hereinabove, result processor  22  generally uses triggers that have the same structure as queries and inferred characteristics of the result set, such as attribute-value pairs. In an embodiment of the present invention, result processor  22  uses triggers of different types, which are particularly appropriate for evaluating result set characteristics, either alone or in combination with query characteristics. Such triggers typically do not include attribute-value pairs, and thus are evaluated independently of the characterization method described hereinabove with reference to  FIG. 2 . For some applications, the implementation of the content presentation rules is modified to accommodate such triggers. 
     Such triggers may include, for example: the result set contains more than, less than, or equal to a threshold number of items; the result set contains more than, less than, or equal to a threshold number of items, and the query contains one or more characteristics. For example, the trigger may be that the result set contains less than 10 items, and the query includes the attribute-value pair brand:Sony; and the result set contains or does not contain one or more particular items. 
     In an embodiment of the present invention, system  22  uses the characterization of a plurality of result sets to alert a manager of the content to potentially interesting patterns in the result sets and/or the search queries. Such alerts may include suggestions to the manager of new rules and/or new triggers. For example, the system may keep track of queries that produce result sets having fewer than a threshold number of items, such as empty result sets, and may notify the manager of frequent queries that are producing such result sets. The system may also notify the manager upon finding that the result sets of certain queries are dominated by certain characteristics that have not yet been included in any rules. Alternatively or additionally, the system creates reports and statistics that are enhanced with inferred characteristics. 
     Reference is now made to  FIG. 3 , which is a flow chart that schematically illustrates a method for performing result-set-based triggering, in accordance with an embodiment of the present invention. This embodiment provides an alternative technique for satisfying triggers, and may be used separately or in combination with the triggering techniques described hereinabove. 
     The method receives as input query  100 , which, as described hereinabove, is typically expressed as attribute-value pairs, attribute-range-value pairs, and/or free text searches. At a search step  102 , search engine  20  applies a search algorithm to search memory  26  for items that satisfy the query, and outputs a query result set  104  for the query, which set contains zero or more items from memory  26 . 
     The method also receives as input a set of at least one trigger  106 , which, as described hereinabove, includes one or more characteristics, which are typically expressed as attribute-value pairs, attribute-range-value pairs, and/or free text searches. At a search step  108 , search engine  20  applies the search algorithm to search memory  26  for items that satisfy trigger  106  when the trigger is regarded as a query, and outputs a trigger result set  110  for the trigger. In other words, search engine  20  interprets trigger  106  as a query. For applications in which trigger  106  comprises a Boolean expression, as described hereinabove with reference to Equation 1, searching memory  26  includes evaluating the Boolean expression, as is known in the search engine art. 
     At an intersection evaluation step  112 , result processor  22  determines an intersection set  114  that includes all items that are found in both query result set  104  and trigger result set  110 . The content manager compares (a) the quotient of an assessment of the number of items in intersection set  114  divided by the number of items in query result set  104  with (b) a threshold  116 . For example threshold  116  may be at least 50%, such as at least 60% or 75%. (Alternatively, the divisor is an assessment of the number of items in trigger result set  110 , or the sum of the assessments of the number of items in the query and trigger result sets  104  and  110 .) The assessment of the number of items is typically performed by counting the number of items, either exactly or approximately. If the quotient is greater than threshold  116 , the result processor determines that trigger  106  is a satisfied trigger  118 . At a rule application step  120 , the system performs the appropriate content presentation action for satisfied trigger  118 , based on the content presentation rules, as described hereinabove, and presents content to the user as specified by the rules, at a content presentation step  122 . 
     Alternatively, at intersection evaluation step  112 , result processor  22  compares (a) an assessment of the number of items in intersection set  114  with (b) threshold  116 . The assessment of the number of items is typically performed by counting the number of items, either exactly or approximately. If the number of items is greater than threshold  116 , the result processor determines that trigger  106  has been satisfied. 
     Result processor  22  repeats steps  108  through  112  to evaluate each of a plurality of triggers  106 . A global threshold  116  may be used for evaluating the plurality of triggers  106 , or a separate trigger may be provided for each of the triggers. 
     For some applications, rather than determine trigger result set  110  separately for each received query  100 , system  22  makes a record of trigger result set  110  and uses the record for evaluating a plurality of queries  100 . For example, system  22  may cache trigger result set  110  for each trigger  106 , and reuse the cached result sets for evaluating a plurality of queries  100 . The cached result sets are recomputed or flushed from the cache upon changes to memory  26  or to trigger  106 . Alternatively, the system makes such a record using an index, a mapping, or a data structure, which provides, for each item in memory  26 , the triggers in whose trigger results the item appears. 
     In some embodiments of the present invention, one or more of the functions described herein as being performed by result processor  22  are instead performed by a module of search engine  20 . 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art, which would occur to persons skilled in the art upon reading the foregoing description.