[DRILL-5774] Excessive memory allocation

• Type: Bug
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 1.11.0
• Component/s: Execution - Relational Operators
• Assignee: Unassigned
• Reporter: Robert Hou
• Created: 07/Sep/17 22:04
• Updated: 26/Feb/19 04:07

Description

This query exhibits excessive memory allocation:

ALTER SESSION SET `exec.sort.disable_managed` = false;
alter session set `planner.width.max_per_node` = 1;
alter session set `planner.disable_exchanges` = true;
alter session set `planner.width.max_per_query` = 1;
select count(*) from (select * from (select id, flatten(str_list) str from dfs.`/drill/testdata/resource-manager/flatten-large-small.json`) d order by d.str) d1 where d1.id=0;

This query does a flatten on a large table. The result is 160M records. Half the records have a one-byte string, and half have a 253-byte string. And then there are 40K records with 223 byte strings.

select length(str), count(*) from (select id, flatten(str_list) str from dfs.`/drill/testdata/resource-manager/flatten-large-small.json`) group by length(str);
+---------+-----------+
| EXPR$0  |  EXPR$1   |
+---------+-----------+
| 223     | 40000     |
| 1       | 80042001  |
| 253     | 80000000  |

From the drillbit.log:

2017-09-02 11:43:44,598 [26550427-6adf-a52e-2ea8-dc52d8d8433f:frag:0:0] DEBUG o.a.d.e.p.i.x.m.ExternalSortBatch - Actual batch schema & sizes {
  str(type: REQUIRED VARCHAR, count: 4096, std size: 54, actual size: 134, data size: 548360)
  id(type: OPTIONAL BIGINT, count: 4096, std size: 8, actual size: 9, data size: 36864)
  Records: 4096, Total size: 1073819648, Data size: 585224, Gross row width: 262163, Net row width: 143, Density: 1}

The data size is 585K, but the batch size is 1 GB. The density is 1%.