Cascalog includes a number of options for the creation and execution of its queries.
<- is the query creation operator. As described in [[How Cascalog executes a query]], <- accepts a vector of output variables and a series of predicates. The various predicates allowed are discussed in [[Guide to custom operations]].
For example, given this dataset:
(def people [["ben" 35]
["jerry" 41]])
The following query filters out all people in the dataset under 40:
(<- [?name ?age]
(people ?name ?age)
(< ?age 40))
| This query does nothing on its own. It can act as a generator for other queries, as described [[here | Guide to custom operations]], or we can bind it to an output tap with the query execution operator. |
?- is the query execution operator. It takes a sequence of <output tap, query> pairs, and executes all supplied queries in parallel. For example:
(?- (stdout)
(<- [?name ?age]
(people ?name ?age)
(< ?age 40)))
Prints the following:
RESULTS
-----------------------
ben 35
-----------------------
(stdout) is a Cascading tap that prints the string representation of output tuples to the output stream.
?<- allows for combined query creation and execution. It accepts a single output tap, a result vector, and a series of predicates, like so:
(?<- (stdout)
[?name ?age]
(people ?name ?age)
(< ?age 40))
This query is functionally equivalent to the previous example, under ?-.
??- accepts any numbers of queries (defined by <-), executes them in parallel, and returns a sequence of sequences of the results of each query’s execution.
That last bit can be a bit confusing. Let’s look at two examples. Here’s the result of executing a single query with ??-:
(def results
(??- (<- [?name ?age]
(people ?name ?age)
(< ?age 40))))
user=> results
((["ben" 35]))
Notice that we have an outer sequence with a single entry, corresponding to the single subquery we executed. This entry is a sequence containing that query’s output tuples. Had we executed two queries, we would see a sequence with two inner sequences:
(def multi-results
(??- (<- [?name ?age]
(people ?name ?age)
(< ?age 40))
(<- [?name ?age]
(people ?name ?age)
(< ?age 50))))
user=> multi-results
((["ben" 35]) (["ben" 35] ["jerry" 41]))
??<- allows for combined query creation and execution into a clojure sequence. Because ??<- only allows execution of a single query, it returns a single sequence of tuples:
(def results-??<-
(??<- [?name ?age]
(people ?name ?age)
(< ?age 50)))
user=> results-??<-
(["ben" 35] ["jerry" 41])
Let us know what was unclear or what has not been covered. Maybe you do not like the guide style or grammar or discover spelling mistakes. Reader feedback is key to making the documentation better.
This documentation site is open source and we welcome pull requests.