Do PreAggregations pull complete data on every refresh interval?

**Problem**

We have introduced cube on postgres , postgres by default won't support complex aggregations without indexes and we have lots of dimensions (country,state,city,town) to plot on.

Data is too big and 3M records and we need to show near real time data.

**Related Cube.js schema**
```javascript
cube(`Cube`, {
pre_aggregations: {
dimensions:... ,
measures: .... ,
mainRollup: {
type: `rollup`,
measureReferences: [count],
dimensionReferences: [country,state,city,town],
timeDimension: order_date,
refreshKey: {
every: `2 seconds`
}
}
}
})
```

Will that execute the complete sql query on postgres on every refresh or it will execute the query that got order_date from last 2 seconds ?