[jira] [Commented] (IMPALA-7559) Inconsistent parquet stat filtering of timestamps at dst change

["Csaba Ringhofer (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/IMPALA-7559?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16610754#comment-16610754 ] 

Csaba Ringhofer commented on IMPALA-7559:
-----------------------------------------

The only solution I see at the moment is to make the accepted interval larger if min/max falls into a hour where local->utc mapping is ambiguous. Min value should be set to the smaller possible UTC value, while max value should be set to the larger one, e.g. predicate 
d = "2017-10-29 02:30:00" (CET) 
should be transformed to stat filter 
 "2017-10-29 00:30:00"  <= d <= "2017-10-29 01:30:00" (UTC). 

I think that CCTZ provides the necessary information, so the only problem is the integration into Impala's Parquet reader logic.

> Inconsistent parquet stat filtering of timestamps at dst change
> ---------------------------------------------------------------
>
>                 Key: IMPALA-7559
>                 URL: https://issues.apache.org/jira/browse/IMPALA-7559
>             Project: IMPALA
>          Issue Type: Bug
>          Components: Backend
>            Reporter: Csaba Ringhofer
>            Priority: Minor
>
> If the min/max value of a timestamp column chunk is during the hour of the Summer->Winter dst change (UTC+2 -> UTC+1 in CET) then stat filtering can drop row groups that contain rows that would be "ok" for the predicate otherwise.
> To reproduce (on current master branch):
> {code}
> 1. it is assumed that the timezone is CET and that flag convert_legacy_hive_parquet_utc_timestamps is enabled
> ( export TZ=CET; bin/start-impala-cluster.py --impalad_args="-convert_legacy_hive_parquet_utc_timestamps=true" )
> 2. create a table in hive and fill data in 3 inserts to create 3 files:
> create table t (i int, d timestamp) stored as parquet;
> insert into t values (1, "2017-10-29 02:30:00"), (2, "2018-10-28 02:30:00");
> insert into t values (3, "2018-10-28 02:30:00");
> insert into t values (4, "2017-10-29 02:30:00")
> 3. Query from Impala
> set num_nodes=1;
> select * from t; -- returns all 4 values (same as Hive) 
> select * from t where d = "2017-10-29 02:30:00"; -- returns 1 in Impala (Hive returns 1,4)
> select * from t where d = "2018-10-28 02:30:00"; -- returns 2 in Impala (Hive returns 2,3)
> profile; -- NumStatsFilteredRowGroups: 2 (only one row group should have been stat filtered)
> select * from t where d = "2018-10-28 02:30:00" or i = 5; -- returns 2 and 3 in Impala (same as Hive), because the "or" part disabled stat filtering
> {code}



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