Pushdown or late materialization?

[xumingming]

I am reading the doc about Aggregation: https://facebookincubator.github.io/velox/develop/aggregations.html

For example, pushdown is possible in the following query:

SELECT a, sum(b) FROM t GROUP BY 1
Pushdown is also possible if the data is filtered using columns other than the column that is the input to the aggregation function. For example, pushdown is enabled in the following query:

SELECT a, sum(b)
FROM t
WHERE a > 100
GROUP BY 1

In these queries, TableScan operator produces “b” column as a LazyVector and “sum” aggregate function loads this vector using ValueHook, e.g. each value is read from the file and passed directly to “sum” aggregate which adds it to the accumulator. No intermediate vector is produced in this case.

I don't quite understand why we say it is a pushdown here, LazyVector is more like a late materialization to me?