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Aggregate function combinators
The name of an aggregate function can have a suffix appended to it. This changes the way the aggregate function works.
-If
The suffix -If can be appended to the name of any aggregate function. In this case, the aggregate function accepts an extra argument – a condition (Uint8 type). The aggregate function processes only the rows that trigger the condition. If the condition was not triggered even once, it returns a default value (usually zeros or empty strings).
Examples: sumIf(column, cond), countIf(cond), avgIf(x, cond), quantilesTimingIf(level1, level2)(x, cond), argMinIf(arg, val, cond) and so on.
With conditional aggregate functions, you can calculate aggregates for several conditions at once, without using subqueries and JOINs. For example, in Yandex.Metrica, conditional aggregate functions are used to implement the segment comparison functionality.
-Array
The -Array suffix can be appended to any aggregate function. In this case, the aggregate function takes arguments of the 'Array(T)' type (arrays) instead of 'T' type arguments. If the aggregate function accepts multiple arguments, this must be arrays of equal lengths. When processing arrays, the aggregate function works like the original aggregate function across all array elements.
Example 1: sumArray(arr) - Totals all the elements of all 'arr' arrays. In this example, it could have been written more simply: sum(arraySum(arr)).
Example 2: uniqArray(arr) – Count the number of unique elements in all 'arr' arrays. This could be done an easier way: uniq(arrayJoin(arr)), but it's not always possible to add 'arrayJoin' to a query.
-If and -Array can be combined. However, 'Array' must come first, then 'If'. Examples: uniqArrayIf(arr, cond), quantilesTimingArrayIf(level1, level2)(arr, cond). Due to this order, the 'cond' argument can't be an array.
-State
If you apply this combinator, the aggregate function doesn't return the resulting value (such as the number of unique values for the uniq function), but an intermediate state of the aggregation (for uniq, this is the hash table for calculating the number of unique values). This is an AggregateFunction(...) that can be used for further processing or stored in a table to finish aggregating later.
To work with these states, use:
- AggregatingMergeTree table engine.
- finalizeAggregation function.
- runningAccumulate function.
- -Merge combinator.
- -MergeState combinator.
-Merge
If you apply this combinator, the aggregate function takes the intermediate aggregation state as an argument, combines the states to finish aggregation, and returns the resulting value.
-MergeState
Merges the intermediate aggregation states in the same way as the -Merge combinator. However, it doesn't return the resulting value, but an intermediate aggregation state, similar to the -State combinator.
-ForEach
Converts an aggregate function for tables into an aggregate function for arrays that aggregates the corresponding array items and returns an array of results. For example, sumForEach for the arrays [1, 2], [3, 4, 5]and[6, 7]returns the result [10, 13, 5] after adding together the corresponding array items.