Hi Ildar,
it seems that we have 2 separate points here:
1) There are bugs in the way we decide which list representation to use
and
2) we could add support for (and an optimized representation for) a list
of a fixed but nullable type.
It seems that - by fixing 1) - we could get rid of the issues you’ve
listed.
But I also think that it would be nice to support lists of a nullable
type (feels like an omission that we don’t support that in the
language) - and potentially provide an efficient representation for
them.
However, it is not clear to me how we would do this.
A few thoughts:
- Would we maintain the current representation for homogenous lists of
non-nullable types?
- Would we introduce a new type tag for “nullable lists”?
- Would we redefine the current representation to mean something else?
Do you have thoughts on those?
Cheers,
Till
On 16 Dec 2015, at 8:12, Ildar Absalyamov wrote:
Hi devs,
Recently I have been playing around with lists and functions, which
receive/return list parameters/values. I have noticed one particular
issue, which seems to be incorrect.
As you might know internally we do support 2 types of lists
homogeneous, where all the items are untagged and the item type is
stored in type definition, and heterogeneous, where items on contrary
are tagged, and the list item type is effectively ANY.
The decision which of two types would be used is usually done by
parser or is altered by IntroduceEnforcedListTypeRule, which
effectively turns heterogenous list into homogenous if all the items
have the same type.
Right now only we allow homogeneous lists to be defined as a field in
some type, we also restrict the item type to be only non-nullable
type:
create type listType {
“id”:int64,
“list”:[int64] // [int64?] is not possible
}
This constraint spans both of the language level as well as
serialization. Under that restriction the only way to load the list,
which contains null values, would be to make the appropriate field
open (open lists are heterogenous by definition).
1) Seems like we’re missing an optimization opportunity when we are
dealing with large sparse lists. Serialization in this case might use
a bit mask to specify which items in the lists are not null, and later
encode only those items.
2) I believe if we alter IntroduceEnforcedListTypeRule to enforce list
to homogeneous list with nullable item type we might resolve issues
https://issues.apache.org/jira/browse/ASTERIXDB-905,
https://issues.apache.org/jira/browse/ASTERIXDB-867,
https://issues.apache.org/jira/browse/ASTERIXDB-1131all at once.
Thoughts?
Best regards,
Ildar
