[IMPALA-7655] Codegen output for conditional functions (if,isnull, coalesce) is very suboptimal

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Component/s: Backend
• Labels: codegen perf performance
• Epic Link: Remove GetCodegendComputeFnWrapper
• Target Version: Product Backlog
• Epic Color: ghx-label-7
• Assignee: Daniel Becker
• Reporter: Tim Armstrong
• Created: 04/Oct/18 18:24
• Updated: 21/Jul/20 09:10
• Resolved: 21/Jul/20 09:10

Description

https://gerrit.cloudera.org/#/c/11565/ provided a clue that an aggregation involving an if() function was very slow, 10x slower than the equivalent version using a case:

[localhost:21000] default> set num_nodes=1; set mt_dop=1; select count(case when l_orderkey is NULL then 1 else NULL end) from tpch10_parquet.lineitem;summary;
NUM_NODES set to 1
MT_DOP set to 1
Query: select count(case when l_orderkey is NULL then 1 else NULL end) from tpch10_parquet.lineitem
Query submitted at: 2018-10-04 11:17:31 (Coordinator: http://tarmstrong-box:25000)
Query progress can be monitored at: http://tarmstrong-box:25000/query_plan?query_id=274b2a6f35cefe31:95a1964200000000
+----------------------------------------------------------+
| count(case when l_orderkey is null then 1 else null end) |
+----------------------------------------------------------+
| 0                                                        |
+----------------------------------------------------------+
Fetched 1 row(s) in 0.51s
+--------------+--------+----------+----------+--------+------------+----------+---------------+-------------------------+
| Operator     | #Hosts | Avg Time | Max Time | #Rows  | Est. #Rows | Peak Mem | Est. Peak Mem | Detail                  |
+--------------+--------+----------+----------+--------+------------+----------+---------------+-------------------------+
| 01:AGGREGATE | 1      | 44.03ms  | 44.03ms  | 1      | 1          | 25.00 KB | 10.00 MB      | FINALIZE                |
| 00:SCAN HDFS | 1      | 411.57ms | 411.57ms | 59.99M | -1         | 16.61 MB | 88.00 MB      | tpch10_parquet.lineitem |
+--------------+--------+----------+----------+--------+------------+----------+---------------+-------------------------+
[localhost:21000] default> set num_nodes=1; set mt_dop=1; select count(if(l_orderkey is NULL, 1, NULL)) from tpch10_parquet.lineitem;summary;
NUM_NODES set to 1
MT_DOP set to 1
Query: select count(if(l_orderkey is NULL, 1, NULL)) from tpch10_parquet.lineitem
Query submitted at: 2018-10-04 11:23:07 (Coordinator: http://tarmstrong-box:25000)
Query progress can be monitored at: http://tarmstrong-box:25000/query_plan?query_id=8e46ab1b84c4dbff:2786ca2600000000
+----------------------------------------+
| count(if(l_orderkey is null, 1, null)) |
+----------------------------------------+
| 0                                      |
+----------------------------------------+
Fetched 1 row(s) in 1.01s
+--------------+--------+----------+----------+--------+------------+----------+---------------+-------------------------+
| Operator     | #Hosts | Avg Time | Max Time | #Rows  | Est. #Rows | Peak Mem | Est. Peak Mem | Detail                  |
+--------------+--------+----------+----------+--------+------------+----------+---------------+-------------------------+
| 01:AGGREGATE | 1      | 422.07ms | 422.07ms | 1      | 1          | 25.00 KB | 10.00 MB      | FINALIZE                |
| 00:SCAN HDFS | 1      | 511.13ms | 511.13ms | 59.99M | -1         | 16.61 MB | 88.00 MB      | tpch10_parquet.lineitem |
+--------------+--------+----------+----------+--------+------------+----------+---------------+-------------------------+

It turns out that this is because we don't have good codegen support for ConditionalFunction, and just fall back to emitting a call to the interpreted path: https://github.com/apache/impala/blob/master/be/src/exprs/conditional-functions.cc#L28

See CaseExpr for an example of much better codegen support: https://github.com/apache/impala/blob/master/be/src/exprs/case-expr.cc#L178

Issue Links

• is part of: IMPALA-7747 Clean up the Expression Rewriter
• is related to: IMPALA-7659 Collect count of nulls when collecting stats