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From "Gianmarco De Francisci Morales (JIRA)" <j...@apache.org>
Subject [jira] Commented: (PIG-1295) Binary comparator for secondary sort
Date Tue, 03 Aug 2010 18:01:46 GMT

    [ https://issues.apache.org/jira/browse/PIG-1295?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12894961#action_12894961
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Gianmarco De Francisci Morales commented on PIG-1295:
-----------------------------------------------------

Thanks for the suggestion Daniel.

My idea to integrate the RawTupleComparator is to modify PigSecondaryKeyComparator to delegate
the comparison to it. I cannot mimic the current behavior of making 2 calls (one for the main
and one for the secondary key) because I do not know the boundaries between the keys. So I
need to make a single call to compare the compound key. There are some issues though.

1) There are some inconsistencies between the behavior of PigTupleRawComparator (the original
one) and PigSecondaryKeyComparator.
Specifically, when tuples are null the first one returns 0 while the second one compares the
indexes.
Furthermore, indexes are compared also when one of the fields in the tuples is null, in order
not to join them (if I understood correctly PIG-927). This is done in PigSecondaryKeyComparator
but not in PigTupleRawComparator.
Is it designed to be like this or is it a bug?
I suppose the behaviors of the two comparators should be more or less the same.

2) In PigSecondaryKeyComparator the key is assumed to be a 2-field tuple where the 0th field
is the main key and the 1st field is the secondary key.
We can directly feed the binary representation of this tuple inside our new raw comparator,
but we need to consolidate the sort orders. Right now there are 2 different and independent
sort orders serialized in the jobConf (pig.sortOrder and pig.secondarySortOrder). In the simple
case, when sort orders for all the columns are specified, we can just concatenate them together
(sort of). There are some problems when we have WholeTuple sort orders as they might differ.

I would like to keep all of this out of the tuple comparator and define some clean interface
to pass the sort orders.  One problem I see is that I probably need the tuple sizes (recursively)
to do this, and this is not known at configuration time. I also need to fix this in the current
comparator, in order to take into account the recursion inside nested tuples.

3) Should I keep all the mIndex/mNull handling outside the RawTupleComparator and write wrappers
that deal with them?
That is, should the RawTupleComparator know how to deal with a NullableTuple or should it
just know its kind of Tuple (BinInterSedes or Default)

> Binary comparator for secondary sort
> ------------------------------------
>
>                 Key: PIG-1295
>                 URL: https://issues.apache.org/jira/browse/PIG-1295
>             Project: Pig
>          Issue Type: Improvement
>          Components: impl
>    Affects Versions: 0.7.0
>            Reporter: Daniel Dai
>            Assignee: Gianmarco De Francisci Morales
>             Fix For: 0.8.0
>
>         Attachments: PIG-1295_0.1.patch, PIG-1295_0.10.patch, PIG-1295_0.11.patch, PIG-1295_0.2.patch,
PIG-1295_0.3.patch, PIG-1295_0.4.patch, PIG-1295_0.5.patch, PIG-1295_0.6.patch, PIG-1295_0.7.patch,
PIG-1295_0.8.patch, PIG-1295_0.9.patch
>
>
> When hadoop framework doing the sorting, it will try to use binary version of comparator
if available. The benefit of binary comparator is we do not need to instantiate the object
before we compare. We see a ~30% speedup after we switch to binary comparator. Currently,
Pig use binary comparator in following case:
> 1. When semantics of order doesn't matter. For example, in distinct, we need to do a
sort in order to filter out duplicate values; however, we do not care how comparator sort
keys. Groupby also share this character. In this case, we rely on hadoop's default binary
comparator
> 2. Semantics of order matter, but the key is of simple type. In this case, we have implementation
for simple types, such as integer, long, float, chararray, databytearray, string
> However, if the key is a tuple and the sort semantics matters, we do not have a binary
comparator implementation. This especially matters when we switch to use secondary sort. In
secondary sort, we convert the inner sort of nested foreach into the secondary key and rely
on hadoop to sorting on both main key and secondary key. The sorting key will become a two
items tuple. Since the secondary key the sorting key of the nested foreach, so the sorting
semantics matters. It turns out we do not have binary comparator once we use secondary sort,
and we see a significant slow down.
> Binary comparator for tuple should be doable once we understand the binary structure
of the serialized tuple. We can focus on most common use cases first, which is "group by"
followed by a nested sort. In this case, we will use secondary sort. Semantics of the first
key does not matter but semantics of secondary key matters. We need to identify the boundary
of main key and secondary key in the binary tuple buffer without instantiate tuple itself.
Then if the first key equals, we use a binary comparator to compare secondary key. Secondary
key can also be a complex data type, but for the first step, we focus on simple secondary
key, which is the most common use case.
> We mark this issue to be a candidate project for "Google summer of code 2010" program.


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