# Summarize

Aggregate and compute

## Signature

```
summarize(operand: Relation, by: AttrList, aggs: Summarization) -> Relation
```

## Examples

```
summarize(shipments, [:sid], total: sum(:qty))
```

```
summarize(shipments, [:sid], total: sum{|t| t.qty * 2 })
```

```
summarize(shipments, [:pid, :qty], {total: sum{|t| t.qty * 2 }}, allbut: true)
```

## Description

Computes the relation obtained by taking the projection of `operand`

on `by`

attributes then extending each tuple `t`

with the result of
aggregations defined by `aggs`

on the tuples from `operand`

matching `t`

.

In SQL terms, `SELECT [by], [agg] FROM operand GROUP BY [by]`

.

## Implementation notes

As of current Alf version, this operator cannot be translated to SQL code. That means that all computations are done in ruby, which may seriously hurt performance.

Similarly, as `summarize`

tends to be a showstopper during compilation, it
is strongly recommended to use it as high as possible in query expressions
trees so as to delegate the largest possible query parts to data engines.