[DRILL-4833] Union-All with a small cardinality input on one side does not get parallelized

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 1.7.0
• Fix Version/s: 1.8.0
• Component/s: Query Planning & Optimization
• Assignee: Aman Sinha
• Reporter: Aman Sinha
• Reviewer: Robert Hou
• Created: 07/Aug/16 20:46
• Updated: 29/Aug/16 17:30
• Resolved: 24/Aug/16 16:35

Description

When a Union-All has an input that is a LIMIT 1 (or some small value relative to the slice_target), and that input is accessing Parquet files, Drill does an optimization where a single Parquet file is read (based on the rowcount statistics in the Parquet file, we determine that reading 1 file is sufficient). This also means that the max width for that major fragment is set to 1 because only 1 minor fragment is needed to read 1 row-group.

The net effect of this is the width of 1 is applied to the major fragment which consists of union-all and its inputs. This is sub-optimal because it prevents parallelization of the other input and the union-all operator itself.

Here's an example query and plan that illustrates the issue:

alter session set `planner.slice_target` = 1;

explain plan for 
(select c.c_nationkey, c.c_custkey, c.c_name
from
dfs.`/Users/asinha/data/tpchmulti/customer` c
inner join
dfs.`/Users/asinha/data/tpchmulti/nation`  n
on c.c_nationkey = n.n_nationkey)

union all

(select c_nationkey, c_custkey, c_name
from dfs.`/Users/asinha/data/tpchmulti/customer` c limit 1)

+------+------+
| text | json |
+------+------+
| 00-00    Screen
00-01      Project(c_nationkey=[$0], c_custkey=[$1], c_name=[$2])
00-02        Project(c_nationkey=[$0], c_custkey=[$1], c_name=[$2])
00-03          UnionAll(all=[true])
00-05            Project(c_nationkey=[$0], c_custkey=[$1], c_name=[$2])
00-07              HashJoin(condition=[=($0, $3)], joinType=[inner])
00-10                Project(c_nationkey=[$0], c_custkey=[$1], c_name=[$2])
00-13                  HashToRandomExchange(dist0=[[$0]])
01-01                    UnorderedMuxExchange
03-01                      Project(c_nationkey=[$0], c_custkey=[$1], c_name=[$2], E_X_P_R_H_A_S_H_F_I_E_L_D=[hash32AsDouble($0)])
03-02                        Scan(groupscan=[ParquetGroupScan [entries=[ReadEntryWithPath [path=file:/Users/asinha/data/tpchmulti/customer]], selectionRoot=file:/Users/asinha/data/tpchmulti/customer, numFiles=1, usedMetadataFile=false, columns=[`c_nationkey`, `c_custkey`, `c_name`]]])
00-09                Project(n_nationkey=[$0])
00-12                  HashToRandomExchange(dist0=[[$0]])
02-01                    UnorderedMuxExchange
04-01                      Project(n_nationkey=[$0], E_X_P_R_H_A_S_H_F_I_E_L_D=[hash32AsDouble($0)])
04-02                        Scan(groupscan=[ParquetGroupScan [entries=[ReadEntryWithPath [path=file:/Users/asinha/data/tpchmulti/nation]], selectionRoot=file:/Users/asinha/data/tpchmulti/nation, numFiles=1, usedMetadataFile=false, columns=[`n_nationkey`]]])
00-04            Project(c_nationkey=[$0], c_custkey=[$1], c_name=[$2])
00-06              SelectionVectorRemover
00-08                Limit(fetch=[1])
00-11                  Scan(groupscan=[ParquetGroupScan [entries=[ReadEntryWithPath [path=/Users/asinha/data/tpchmulti/customer/01.parquet]], selectionRoot=file:/Users/asinha/data/tpchmulti/customer, numFiles=1, usedMetadataFile=false, columns=[`c_nationkey`, `c_custkey`, `c_name`]]])

Note that Union-all and HashJoin are part of fragment 0 (single minor fragment) even though they could have been parallelized. This clearly affects performance for larger data sets.

Issue Links

• links to: GitHub Pull Request #562
• links to: GitHub Pull Request #566