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From "Taylor, Ronald (Ronald)" <Ronald.Tay...@cchmc.org>
Subject hint to use a global index is not working - need to find out why
Date Thu, 19 Apr 2018 13:31:29 GMT
Hello Phoenix users,

I am a novice Phoenix user and this is my first post to this user list. I did some searching
in the list archives, but could not find an answer to what I hope is a simple question: my
global index is being ignored, even after I add a Hint, and I want to know why.

We are using Phoenix 4.7 in the Hortonworks distribution. Looks like Hortonworks has been
backporting at least some phoenix updates into their version of phoenix 4.7, so I guess it
is a custom distribution. See

     https://docs.hortonworks.com/HDPDocuments/HDP2/HDP-2.6.4/bk_release-notes/content/patch_phoenix.html

I have created a simple table of about 8 million rows, and about 15 columns, with several
fields having global indexes. I created the main table (variantjoin_rtsalted24) and its indexes,
and then used a bulk loader to populate them from a tab-delimited file. That appeared to work
fine.

chrom_int is one field on which there is a global index, named vj2_chrom_int. And you can
see the index being automatically being used below, where it is the only field being returned.
Time required is 0.124 sec.

0: jdbc:phoenix:> SELECT VJ.chrom_int  FROM VARIANTJOIN_RTSALTED24 as VJ WHERE (chrom_int
=18 ) limit 5;

+------------+

| CHROM_INT  |

+------------+

| 18         |

| 18         |

| 18         |

| 18         |

| 18         |

+------------+

5 rows selected (0.124 seconds)

0: jdbc:phoenix:>

You can see that the vj2_chrom_int index is automatically being used, as I understand things
 by the "RANGE SCAN" wording and "[0,1" in the explain plan:

0: jdbc:phoenix:> explain SELECT VJ.chrom_int  FROM VARIANTJOIN_RTSALTED24 as VJ WHERE
(chrom_int =18 ) limit 5;

+------------------------------------------------------------------------------+

|                                           PLAN                               |

+------------------------------------------------------------------------------+

| CLIENT 24-CHUNK SERIAL 24-WAY ROUND ROBIN RANGE SCAN OVER VJ2_CHROM_INT [0,1 |

|     SERVER FILTER BY FIRST KEY ONLY                                          |

|     SERVER 5 ROW LIMIT                                                       |

| CLIENT 5 ROW LIMIT                                                           |

+------------------------------------------------------------------------------+

4 rows selected (0.043 seconds)

0: jdbc:phoenix:>


I can use a Hint to tell Phoenix to NOT use this index, as seen below. And that increases
the time needed to 1.97 sec, over an order of magnitude more time than the 0.124 sec required
with index use.

0: jdbc:phoenix:> SELECT /*+ NO_INDEX */ VJ.chrom_int  FROM VARIANTJOIN_RTSALTED24 as VJ
WHERE (chrom_int =18 ) limit 5;

+------------+

| CHROM_INT  |

+------------+

| 18         |

| 18         |

| 18         |

| 18         |

| 18         |

+------------+

5 rows selected (1.977 seconds)

0: jdbc:phoenix:>

And here is the explain plan for that:


0: jdbc:phoenix:> explain SELECT /*+ NO_INDEX */ VJ.chrom_int  FROM VARIANTJOIN_RTSALTED24
as VJ WHERE (chrom_int =18 ) limit 5;

+------------------------------------------------------------------------------+

|                                                        PLAN                  |

+------------------------------------------------------------------------------+

| CLIENT 72-CHUNK 14325202 ROWS 15099524157 BYTES PARALLEL 24-WAY ROUND ROBIN  |

|     SERVER FILTER BY CHROM_INT = 18                                          |

|     SERVER 5 ROW LIMIT                                                       |

| CLIENT 5 ROW LIMIT                                                           |

+------------------------------------------------------------------------------+

4 rows selected (0.009 seconds)

Now,  I want to add other fields for retrieval. For example, "genomic_range". The Phoenix
documentation says in such a case I must add a Hint to force Phoenix to make use of the index
(since it is a simple global index, not a covered index wherein genomic_range has been added.)
So I tried that. See below. Alas, the response time is about the same as what I get with NO_INDEX.
 It appears that, even with the Hint, the index is not being used.

0: jdbc:phoenix:> SELECT/*+ INDEX(VJ jv2_chrom_int) */ VJ.chrom_int, genomic_range  FROM
VARIANTJOIN_RTSALTED24 as VJ WHERE (chrom_int =18 ) limit 5;
+------------+----------------------+
| CHROM_INT  |    GENOMIC_RANGE     |
+------------+----------------------+
| 18         | [49546,49547)        |
| 18         | [20003625,20003626)  |
| 18         | [19618749,19618752)  |
| 18         | [47561,47583)        |
| 18         | [20024261,20024272)  |
+------------+----------------------+
5 rows selected (1.799 seconds)
0: jdbc:phoenix:>


And below is the explain plan for the query with the index failure. No indication of index
use, that I can tell.

0: jdbc:phoenix:> explain SELECT/*+ INDEX(VJ jv2_chrom_int) */ VJ.chrom_int, genomic_range
 FROM VARIANTJOIN_RTSALTED24 as VJ WHERE (chrom_int =18 ) limit 5;

+------------------------------------------------------------------------------+
|                                                        PLAN                  |
+------------------------------------------------------------------------------+
| CLIENT 72-CHUNK 14325202 ROWS 15099524157 BYTES PARALLEL 24-WAY ROUND ROBIN  |
|     SERVER FILTER BY CHROM_INT = 18                                          |
|     SERVER 5 ROW LIMIT                                                       |
| CLIENT 5 ROW LIMIT                                                           |
+------------------------------------------------------------------------------+
4 rows selected (0.042 seconds)
0: jdbc:phoenix:>

So I am puzzled and frustrated. How do I get my index Hint to work?  The difference in timing
between automatic use (when the index is the only field being retrieved) and when NO_INDEX
is used tells me that the index table is there and can indeed be used. But something is going
wrong when I try to force its use via a Hint. Guidance would be very much appreciated on this
basic point.

  - Ron Taylor
Ronald C. Taylor, Ph.D.
Divisions of Immunobiology and Biomedical Informatics
Cincinnati Children's Hospital Medical Center
Office phone: 513-803-4880
Cell phone:     509-783-7308
Email: ronald.taylor@cchmc.org<mailto:ronald.taylor@cchmc.org>


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