> ## Documentation Index
> Fetch the complete documentation index at: https://doc.lucidworks.com/llms.txt
> Use this file to discover all available pages before exploring further.

# SQL Aggregations

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[localhost link]: http://localhost:3000/docs/4/fusion-ai/concepts/signals-and-aggregations/aggregations/sql-aggregations

[mintlify link]: https://doc.lucidworks.com/docs/4/fusion-ai/concepts/signals-and-aggregations/aggregations/sql-aggregations

[old doc.lw link]: https://doc.lucidworks.com/fusion-ai/4.2/471

SQL aggregation is used for aggregating signals or other data. SQL is a familiar query language that is well suited to data aggregation.

<Note>
  The aggregation approach available in prior Fusion releases is still available, though it is deprecated. We now refer to the [prior aggregation approach](/docs/4/fusion-ai/concepts/signals-and-aggregations/aggregations/legacy-aggregations) as "legacy aggregations."
</Note>

<LwTemplate />

## Advantages of SQL aggregation

These are advantages of SQL aggregation relative to legacy aggregation:

* **It is SQL!.** You can write SQL queries to aggregate your data.

* **Built-in aggregation functions.** A SQL query can use any of the functions provided by [Spark SQL](https://spark.apache.org/docs/latest/api/scala/index.html#org.apache.spark.sql.functions\$). Use these functions to perform complex aggregations or to enrich aggregation results.

* **A customizable time-decay function.** You can now customize the [exponential time-decay function](https://en.wikipedia.org/wiki/Exponential_decay) that Fusion uses for aggregations.\
  The time-decay function is implemented as a UDAF, so you can easily implement your own time-decay function.

* **Aggregate data from many types of data sources.** You can use any asset in the Fusion Catalog as a data source. This lets you aggregate data from any data source supported by Spark.

* **Performance.** Although performance results can vary, Fusion SQL aggregations are roughly 5 times faster than legacy Fusion aggregations (using the default aggregation as the comparison).

## Key features

### Rollup SQL

Most aggregation jobs run with the catch-up flag set to `true`, which means that Fusion only computes aggregations for new signals that have arrived *since* the last time the job was run, and *up to and including* `ref_time`, which is usually the run time of the current job. Fusion must "roll up" the newly aggregated rows into any existing aggregated rows in the `_aggr` collection.

If you are using SQL to do aggregation but have not supplied a custom rollup SQL, Fusion generates a basic rollup SQL script automatically by consulting the schema of the aggregated documents. If your rollup logic is complex, you can provide a custom rollup SQL script.

Fusion’s basic rollup is a SUM of numeric types (long, integer, double, and float) and can support `time_decay` for the weight field. If you are not using `time_decay` in your weight calculations, then the weight is calculated using `weight_d`. If you do include `time_decay` with your weight calculations, then the weight is calculated as a combination of timestamp, `halfLife`, `ref_time`, and `weight_d`.

The basic rollup SQL can be grouped by `DOC_ID`, `QUERY`, `USER_ID`, and `FILTERS`. The last GROUP BY field in the main SQL is used so it will ultimately group any newly aggregated rows with existing rows.

This is an example of a rollup query:

```sql wrap  theme={"dark"}
SELECT query_s, doc_id_s, time_decay(1, timestamp_tdt, "30 days", ref_time, weight_d)
AS weight_d , SUM(aggr_count_i) AS aggr_count_i
FROM `commerce_signals_aggr` GROUP BY query_s, doc_id_s
```

### Time-range filtering

When Fusion rolls up new data into an aggregation, time-range filtering lets you ensure that Fusion does not aggregate the same data over and over again.

Fusion applies a time-range filter when loading rows from Solr, before executing the aggregation SQL statement. In other words, the SQL executes over rows that are already filtered by the appropriate time range for the aggregation job.

Notice that the examples [Perform the Default SQL Aggregation](#rollup-sql) and [Use Different Weights Based on Signal Types](#weight-aggregated-values-using-a-time-decay-function) do not include a time-range filter. Fusion computes the time-range filter automatically as follows:

* If the catch-up flag is set to `true`, Fusion uses the last time the job was run and `ref_time` (which you typically set to the current time). This is equivalent to the WHERE clause `WHERE time > last_run_time AND time <= ref_time`.
* If the catch-up flag is not set to `true`, Fusion uses a filter with `ref_time` (and no start time). This is equivalent to the WHERE clause `WHERE time <= ref_time`.

The built-in time logic should suffice for most use cases. You can set the time range filter to **TO** and specify a WHERE clause filter to achieve more complex time based filtering.

Time range in Fusion is equivalent to [date range in Solr](https://solr.apache.org/guide/8_8/working-with-dates.html).

**Example values for time range:**

* `[* TO NOW]` - all past events
* `[2000-11-01 TO 2020-12-01]` – specify by exact date
* `[* TO 2020-12-01]` – from the first data point to the end of the day specified
* `[2000 TO 2020]` - from the start of a year to the end of another year

### SQL functions

A Spark SQL aggregation query can use any of the functions provided by [Spark SQL](https://spark.apache.org/docs/latest/api/scala/index.html#org.apache.spark.sql.functions\$). Use these functions to perform complex aggregations or to enrich aggregation results.

### Weight aggregated values using a time-decay function

Fusion automatically uses a default `time_decay` function to compute and apply appropriate weights to aggregation groups during aggregation. Larger weights are assigned to more recent events. This reduces the impact of less-recent signals. Intuitively, older signals (and the user behavior they represent) should count less than newer signals.

If the default `time_decay` function does not meet your needs, you can modify it. The `time_decay` function is implemented as a [UserDefinedAggregateFunction](https://spark.apache.org/docs/latest/api/scala/index.html#org.apache.spark.sql.expressions.UserDefinedAggregateFunction) (UDAF).

**Full function signature**

This is the UDAF signature of the default `time_decay` function:

```js wrap  theme={"dark"}
time_decay(count: Long,
           timestamp: Timestamp,
           halfLife: String (calendar interval),
           ref_time: Timestamp,
           weight_d: Double)
```

One small difference between the prior and current behavior is worth mentioning in passing:

* Prior to Release 4.0, the `decay_sum` aggregator function used the difference between the `aggregationTime` (the time at which the aggregation job is run) and the event time to calculate exponentially decayed numerical values.
* In Release 4.0, `time_decay` is a similar function. In `time_decay`, `ref_time` is used instead of `aggregationTime`. You can set `aggregationTime` to some other time than the run time of the aggregation job.

In practice, you will probably want to use `aggregationTime` as `ref_time`.

**Abbreviated function signature and default values**

Your function call can also use this abbreviated UDAF signature, that omits `halfLife`, `ref_time`, and `weight_d`:

```js wrap  theme={"dark"}
time_decay(count: Long,
           timestamp: Timestamp)
```

In this case, Fusion fills in these values for the omitted parameters: `halfLife` = `30 days`, `ref_time` = `NOW`, and `weight_d` = `0.1`.

**Matching legacy aggregation**

To match the results of legacy aggregation, either use the abbreviated function signature or supply these values for the mentioned parameters: `halfLife` = `30 days`, `ref_time` = `NOW`, and `weight_d` = `0.1`.

**Parameters**

Parameters for `time_decay` are:

| Parameter   | Description                                                                                                                                                                                                                                                                                                                               |
| ----------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `count`     | Number of occurrences of the event. Typically, the increment is 1, though there is no reason it could not be some other number. In most cases, you simply pass `count_i`, which is the event count field used by Fusion signals, as shown in the [SQL aggregation examples](#sql-functions).                                              |
| `timestamp` | The date-and-time for the event. This time is the beginning of the interval used to calculate the time-based decay factor.                                                                                                                                                                                                                |
| `halfLife`  | Half life for the exponential decay that Fusion calculates. It is some interval of time, for example, `30 days` or `10 minutes`. The `interval` prefix is optional. Fusion treats `30 days` as equivalent to `interval 30 days`.                                                                                                          |
| `ref_time`  | Reference time used to compute the age of an event for the time-decay computation. It is usually the time when the aggregation job runs (`NOW`). The reference time is not present in the data; Fusion determines the reference time at runtime. Fusion automatically attaches a `ref_time` column to every row before executing the SQL. |
| `weight_d`  | Initial weight for an event, prior to the decay calculation. This value is typically not present in the signal data.<br />  You can use SQL to compute `weight_d`; see [Use Different Weights Based on Signal Types](#weight-aggregated-values-using-a-time-decay-function) for an example.                                               |

**Sample calculation of the age of a signal**

This is an example of how Fusion calculates the age of a signal:

Imagine a SQL aggregation job that runs at `Tuesday, July 11, 2017 1:00:00 AM (1499734800)`.
For a signal with the timestamp `Tuesday, July 11, 2017 12:00:00 AM (1499731200)`, the age of the signal in relation to the reference time is 1 hour.