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From Sumit Nigam <sumit_o...@yahoo.com>
Subject Re: Help with LIMIT clause
Date Fri, 11 Dec 2015 03:30:15 GMT
Thank you James.
I am using Phoenix 4.5.1 with HBase-0.98.14.
I am also noticing that if WHERE clause returns a fewer number of records, then ORDER BY with
LIMIT works fine. Does this input help in any way?
I will file a CR.
Thanks again,Sumit
      From: James Taylor <jamestaylor@apache.org>
 To: user <user@phoenix.apache.org>; Sumit Nigam <sumit_only@yahoo.com> 
 Sent: Thursday, December 10, 2015 11:34 PM
 Subject: Re: Help with LIMIT clause
   
Hi Sumit,I agree, these two queries should return the same result, as long as you have the
ORDER BY clause. What version of Phoenix are you using? What does your DDL look like? Please
file a JIRA that ideally includes a way of reproducing the issue.
select current_timestamp from TBL order by current_timestamp desc limit 1;select max(current_timestamp)
from TBL;

Thanks,James



On Thu, Dec 10, 2015 at 8:58 AM, Sumit Nigam <sumit_only@yahoo.com> wrote:

In thinking a bit more about it, this should be a bug in Phoenix. This is because even with
LIMIT clause I have a order by timestamp DESC, which means that column values MUST have been
sorted prior to applying LIMIT clause. The LIMIT should then give a MAX value in such a case.
Also, surprisingly this only seems to be failing in cases where there are huge number of records.
Like, in my case I have 200K + records.
Any help will be appreciated.
Thanks,Sumit
      From: Sumit Nigam <sumit_only@yahoo.com>
 To: Users Mail List Phoenix <user@phoenix.apache.org> 
 Sent: Thursday, December 10, 2015 8:05 PM
 Subject: Help with LIMIT clause
  
Hi,
The link for salted tables https://phoenix.apache.org/salted.html mentions "Since salting
table would not store the data sequentially, a strict sequential scan would not return all
the data in the natural sorted fashion. Clauses that currently would force a sequential scan,
for example, clauses with LIMIT, would likely to return items that are different from a normal
table"
So, would a simple query such as SELECT CURRENT_TIMESTAMP FROM TBL ORDER BY CURRENT_TIMESTAMP DESC
LIMIT 1; not really return the MAX(CURRENT_TIMESTAMP) ?
PK is on 2 columns with CURRENT_TIMESTAMP as the leading one. I am noticing this issue:
select current_timestamp from TBL order by current_timestamp desc limit 1;+------------------------------------------+|
           CURRENT_TIMESTAMP             |+------------------------------------------+|
1448815328556                            |+------------------------------------------+

select max(current_timestamp) from TBL;+------------------------------------------+|    
    MAX("CURRENT_TIMESTAMP")         |+------------------------------------------+|
1449732792090                            |+------------------------------------------+
The results are different. MAX is of course, returning the right record.
If this is the case, then what should be done where LIMIT is really to be used? What can I
replace it with to get the desired behavior?
Is this also correct that when there is a WHERE clause limiting the number of projected records,
then LIMIT seems to work fine? I seem to be noticing that also.
This is with hbase 0.98.14 and phoenix 4.5.x
Thanks,Sumit

  



 
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