westonpace commented on issue #38837:
URL: https://github.com/apache/arrow/issues/38837#issuecomment-2125124495
> And just to be clear "up front" means at the start of the stream, not
per-batch, right?
Apologies if I am misunderstanding but, given David's point here, aren't we
talking about two different API calls? For example, from the perspective of
duckdb it is often something like (this is maybe over-simplifying things)...
```
left_stats = get_statistics(left_table.join_id, filter)
right_stats = get_statistics(right_table.join_id, filter)
if left_stats.approx_count_distinct < right_stats.approx_count_distinct:
hashtable = make_hashtable(load_array(left_table.join_id))
probe(hashtable, right_table)
else:
hashtable = make_hashtable(load_array(right_table.join_id))
probe(hashtable, left_table)
```
In other words, first I get the statistics (and no data), then I do some
optimization / calculation with the statistics, and then I might choose to
actually load the data.
If these are two separate API calls then the statistics are just another
record batch. What is the advantage of making a single stream / message that
contains both data and statistics?
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