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From "Gianmarco De Francisci Morales (JIRA)" <j...@apache.org>
Subject [jira] Commented: (PIG-1295) Binary comparator for secondary sort
Date Fri, 16 Jul 2010 17:18:51 GMT

    [ https://issues.apache.org/jira/browse/PIG-1295?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12889231#action_12889231
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Gianmarco De Francisci Morales commented on PIG-1295:
-----------------------------------------------------

In DataType the type bytes are sorted in such a way that the comparison between different
data types yields a standard order. This is achieved by carefully assigning the byte values
to the types.
In BinInterSedes this does not happen. So, to reproduce the same order, I need to sort the
bytes somehow. The easiest way is to reassign the values in a way that is coherent with DataType.
The hard way would be to implement a comparison method with all the possible combinations
taken into account, but this is crazy to maintain.
I have also the same problem for costants: because for INTEGER_0/1 and BOOLEAN_TRUE/FALSE
there is no value to read, and the two data type bytes are different, with the current design
I need to ensure that BOOLEAN_TRUE > BOOLEAN_FALSE and INTEGER_1 > INTEGER_0.
Furthermore, It would be good to sort the byte types so that INTEGER > INTEGER_INSHORT
> INTEGER_INBYTE etc...

> 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_0.2.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
>
>
> 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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