Aman Sinha created IMPALA-10116:
-----------------------------------
Summary: Builtin cast function's selectivity is different from
that of explicit cast
Key: IMPALA-10116
URL: https://issues.apache.org/jira/browse/IMPALA-10116
Project: IMPALA
Issue Type: Improvement
Components: Frontend
Affects Versions: Impala 3.4.0
Reporter: Aman Sinha
Assignee: Aman Sinha
Query 1 below uses 'casttobigint()' in the IS NOT NULL predicate and its
selectivity is computed as the default 10% of the input rows, resulting in
cardinality = 7.3K. The predicate in Query 2 with 'CAST' expr computes the
correct cardinality of 73.05K.
Query 1:
{noformat}
Query: explain select * from date_dim d1, date_dim d2 where d1.d_week_seq =
d2.d_week_seq - 52 and casttobigint(d1.d_week_seq) is not null and
casttobigint(d2.d_week_seq) is not null
|
| 00:SCAN HDFS [tpcds.date_dim d1] |
| HDFS partitions=1/1 files=1 size=9.84MB |
| predicates: casttobigint(d1.d_week_seq) IS NOT NULL |
| runtime filters: RF000 -> d1.d_week_seq |
| row-size=255B cardinality=7.30K |
+-------------------------------------------------------------+
{noformat}
Query 2:
{noformat}
Query: explain select * from date_dim d1, date_dim d2 where d1.d_week_seq =
d2.d_week_seq - 52 and cast(d1.d_week_seq as bigint) is not null and
cast(d2.d_week_seq as bigint) is not null
{noformat} |
| 00:SCAN HDFS [tpcds.date_dim d1] |
| HDFS partitions=1/1 files=1 size=9.84MB |
| predicates: CAST(d1.d_week_seq AS BIGINT) IS NOT NULL |
| runtime filters: RF000 -> d1.d_week_seq |
| row-size=255B cardinality=73.05K |
+-------------------------------------------------------------+
{noformat}
Query 1 should ideally provide the same cardinality as Query 2. Note that I
had to comment out the following lines in FunctionCallExpr.java because a user
query is not supposed to directly call the builtin cast function. However, for
an external frontend module that calls functions in impala-frontend.jar, this
is supported and we should make the behavior consistent.
{noformat}
+// if (isBuiltinCastFunction()) {
+// throw new AnalysisException(toSql() +
+// " is reserved for internal use only. Use 'cast(expr AS type)'
instead.");
+// }
{noformat}
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