Document top operator (#5276)

docs/language/operators/README.md

@@ -31,6 +31,7 @@ operators listed below, Zed also allows for the creation of
 * [summarize](summarize.md) -  perform aggregations
 * [switch](switch.md) -  route values based on cases
 * [tail](tail.md) - copy trailing values of input sequence
+* [top](top.md) - get top N sorted values of input sequence
 * [uniq](uniq.md) - deduplicate adjacent values
 * [where](where.md) - select values based on a Boolean expression
 * [yield](yield.md) - emit values from expressions

docs/language/operators/top.md

@@ -0,0 +1,42 @@
+### Operator
+
+  **top** — get top N sorted values of input sequence
+
+### Synopsis
+
+```
+top <const-expr> <expr> [, <expr> ...]
+```
+### Description
+
+The `top` operator returns the top N values from a sequence sorted in descending
+order by one or more expressions. N is given by `<const-expr>`, a compile-time
+constant expression that evaluates to a positive integer.
+
+`top` is functionally similar to [`sort`](sort.md) but is less resource
+intensive because only the top N values are stored in memory (i.e., values
+less than the minimum are discarded).
+
+### Examples
+
+_Grab the top two values from a sequence of integers_
+```mdtest-command
+echo '1 5 3 9 23 7' | zq -z 'top 2 this' -
+```
+=>
+```mdtest-output
+23
+9
+```
+_Find the two names most frequently referenced in a sequence of records_
+```mdtest-command
+echo '{name:"joe", age:22} {name:"bob", age:37} {name:"liz", age:25}
+      {name:"bob", age:18} {name:"liz", age:34} {name:"zoe", age:55}
+      {name:"ray", age:44} {name:"sue", age:41} {name:"liz", age:60}' |
+  zq -z 'count() by name | top 2 count' -
+```
+=>
+```mdtest-output
+{name:"liz",count:3(uint64)}
+{name:"bob",count:2(uint64)}
+```