[IMPALA-8647] The cardinality associated with a SelectNode is 0 even if its child node has a non-zero cardinality

• Type: Bug
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Fix Version/s: Impala 3.4.0
• Component/s: Frontend
• Labels: planner
• Epic Color: ghx-label-2
• Assignee: Tim Armstrong
• Reporter: Fang-Yu Rao
• Created: 11/Jun/19 05:10
• Updated: 14/May/20 03:31
• Resolved: 26/Sep/19 23:08

Description

Consider the following nested SQL statement.

EXPLAIN SELECT *
FROM functional_parquet.alltypessmall WHERE 1 IN
(SELECT int_col FROM functional_parquet.alltypestiny LIMIT 1);

It seems that the cardinality of the SelectNode should be 1 instead of 0. Specifically, if we had executed

compute stats functional_parquet.alltypestiny

before issuing this SQL statement at the very beginning, the returned cardinality of this SelectNode would be 1 instead of 0. Not very sure if this is a bug. It looks like the cardinality of a SelectNode depends on whether there is stats information associated with its child node. The cardinality of a SelectNode would still be 0 even if its child node (ExchangeNode in this case) has a non-zero cardinality.