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over

Operator

over traverse nested values as a lateral query

Synopsis

over <expr> [, <expr>...]
over <expr> [, <expr>...] [with <var>=<expr> [, ... <var>[=<expr>]] => ( <lateral> )

The over operator traverses complex values to create a new sequence of derived values (e.g., the elements of an array) and either (in the first form) sends the new values directly to its output or (in the second form) sends the values to a scoped computation as indicated by <lateral>, which may represent any Zed subquery operating on the derived sequence of values as this.

Each expression <expr> is evaluated in left-to-right order and derived sequences are generated from each such result depending on its types:

  • an array value generates each of its element,
  • a map value generates a sequence of records of the form {key:<key>,value:<value>} for each entry in the map, and
  • all other values generate a single value equal to itself.

Records can be converted to maps with the flatten function resulting in a map that can be traversed, e.g., if this is a record, it can be traversed with over flatten(this).

The nested subquery depicted as <lateral> is called a "lateral query" as the outer query operates on the top-level sequence of values while the lateral query operates on subsequences of values derived from each input value. This pattern rhymes with the SQL pattern of a "lateral join", which runs a SQL subquery for each row of the outer query's table.

In a Zed lateral query, each input value induces a derived subsequence and for each such input, the lateral query runs to completion and yields its results. In this way, operators like sort and summarize, which operate on their entire input, run to completion for each subsequence and yield to the output the lateral result set for each outer input as a sequence of values.

Within the lateral query, this refers to the values of the subsequence thereby preventing lateral expressions from accessing the outer this. To accommodate such references, the over operator includes a with clause that binds arbitrary expressions evaluated in the outer scope to variables that may be referenced by name in the lateral scope.

Note that any such variable definitions override implied field references of this. If a both a field named "x" and a variable named "x" need be referenced in the lateral scope, the field reference should be qualified as this.x while the variable is referenced simply as x.

Lateral queries may be nested to arbitrary depth and accesses to variables in parent lateral query bodies follows lexical scoping.

Examples

Over evaluates each expression and emits it

echo null | zq -z 'over 1,2,"foo"' -

=>

1
2
"foo"

The over clause is evaluated once per each input value

echo "null null" | zq -z 'over 1,2' -

=>

1
2
1
2

Array elements are enumerated

echo null | zq -z 'over [1,2],[3,4,5]' -

=>

1
2
3
4
5

Over traversing an array

echo '{a:[1,2,3]}' | zq -z 'over a' -

=>

1
2
3

Filter the traversed values

echo '{a:[6,5,4]} {a:[3,2,1]}' | zq -z 'over a | this % 2 == 0' -

=>

6
4
2

Aggregate the traversed values

echo '{a:[1,2]} {a:[3,4,5]}' | zq -z 'over a | sum(this)' -

=>

{sum:15}

Aggregate the traversed values in a lateral query

echo '{a:[1,2]} {a:[3,4,5]}' | zq -z 'over a => ( sum(this) )' -

=>

{sum:3}
{sum:12}

Access the outer values in a lateral query

echo '{a:[1,2],s:"foo"} {a:[3,4,5],s:"bar"}' | zq -z 'over a with s => (sum(this) | yield {s,sum})' -

=>

{s:"foo",sum:3}
{s:"bar",sum:12}

Traverse a record by flattening it

echo '{s:"foo",r:{a:1,b:2}} {s:"bar",r:{a:3,b:4}} ' | zq -z 'over flatten(r) with s => (yield {s,key:key[0],value})' -

=>

{s:"foo",key:"a",value:1}
{s:"foo",key:"b",value:2}
{s:"bar",key:"a",value:3}
{s:"bar",key:"b",value:4}