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From Sergey Soldatov <sergeysolda...@gmail.com>
Subject Re: SELECT + ORDER BY vs self-join
Date Tue, 31 Oct 2017 07:02:54 GMT
I agree with James that this happens because the index was not involved
because it doesn't cover all columns. I believe that in the second case,
the RHT  is using the index to create a list of rowkeys and they are used
for point lookups by skipscan.

bq. When is using the self-join a worse choice than the simple select?

Hash join has it's own limitations:
1.  RHT is supposed to be small, so it's better to keep LIMIT small (far
less than 30 mil).
2.  Client is always involved to collect data from RHT, build the hash join
cache and send it to all RSes.


bq. Is there a better way to construct this query?

Using local index may help in this case.

Thanks,
Sergey


On Mon, Oct 30, 2017 at 11:26 PM, James Taylor <jamestaylor@apache.org>
wrote:

> Please file a JIRA and include the explain plan for each of the queries. I
> suspect your index is not being used in the first query due to the
> selection of all the columns. You can try hinting the query to force your
> index to be used. See https://phoenix.apache.org/secondary_indexing.html#
> Index_Usage
>
> Thanks,
> James
>
> On Mon, Oct 30, 2017 at 7:02 AM, Marcin Januszkiewicz <
> januszkiewicz.marcin@gmail.com> wrote:
>
>> We have a wide table with 100M records created with the following DDL:
>>
>> CREATE TABLE traces (
>>   rowkey VARCHAR PRIMARY KEY,
>>   time VARCHAR,
>>   number VARCHAR,
>>   +40 more columns)
>>
>> We want to select a large (~30M records) subset of this data with the
>> query:
>>
>> SELECT *all columns*
>>   FROM traces
>>   WHERE (UPPER(number) LIKE 'PO %')
>>   ORDER BY time DESC, ROWKEY
>>   LIMIT 101;
>>
>> This times out after 15 minutes and puts a huge load on our cluster.
>> We have an alternate way of selecting this data:
>>
>> SELECT t.rowkey, *all columns*
>> FROM TRACES t
>> JOIN (
>>   SELECT rowkey
>>   FROM TRACES
>>   WHERE (UPPER(number) LIKE 'PO %')
>>   ORDER BY time DESC, ROWKEY
>>   LIMIT 101
>> ) ix
>> ON t.ROWKEY = ix.ROWKEY
>> order by t.ROWKEY;
>>
>> Which completes in just under a minute.
>> Is there a better way to construct this query?
>> When is using the self-join a worse choice than the simple select?
>> Given that we have a functional index on UPPER(number), could this
>> potentially be a statistics-based optimizer decision?
>>
>> --
>> Pozdrawiam,
>> Marcin Januszkiewicz
>>
>
>

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