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From Anoop Sam John <anoo...@huawei.com>
Subject Regarding order of filters in FilterList
Date Tue, 05 Jun 2012 04:36:44 GMT
Hi All
               One thing came while going through Filter code

Suppose I am using a FilterList along with my Scan. The list contains one PageFilter(max pages=N)
and one SingleColumnValueFilter.[One filter checks a col value and other deals with number
of rows in result] So as a user what I expect out of this usage is to get N number of rows
where colval=X
Now if I create my FilterList like below things would work fine

FilterList list = new FilterList();
SingleColumnValueFilter f = new SingleColumnValueFilter(..)
list.add( f);
list.add( new PageFilter(..));

Just use the code with slight diff in the order in which the filters are added
FilterList list = new FilterList();
list.add( new PageFilter(..));
SingleColumnValueFilter f = new SingleColumnValueFilter(..)
list.add( f);

Being a user I would expect to get the same result. But it may not be. This even can return
me empty results also.
Here as the filter which deals with the number of returned rows coming 1st. So even if the
second filter might filter out one KV or row later, this number tracked within the filter
getting incremented. [1st filter never gets a chance to rollback the operation when the next
filter filters out the row/KV]

If we use like the 1st way in which the filters dealing with the number of rows or KVs as
the last items in the FilterList, there wont be any problem. Do some one feel this as an issue
with our filter framework and FilterList ?  Atleast we should document this clearly I think.
Pls give your suggestion.

Note : In the above shown code sample if we are going with latet version only=true for SingleColumnValueFilter
, even the second scenario also will be working fine. Here the diff would be skipping one
row will be handled by the filterRowKey() itself and filterRow() wont get called for the ignored
rowkey.[PageFilter deals with filterRow()]
So it all depends on which of the APIs the filter is using to decide to include or skip or
seek to rows.

Also there is  one issues opened regarding the FilterList  HBASE-6132    Pls give your valuable
thoughts and suggestions


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