[jira] [Commented] (HIVE-20574) Column statistics give erraneous numDistinct

["Ajay Jadhav (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HIVE-20574?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16624054#comment-16624054 ] 

Ajay Jadhav commented on HIVE-20574:
------------------------------------

Hive exposes this setting- [https://cwiki.apache.org/confluence/display/Hive/Configuration+Properties#ConfigurationProperties-hive.stats.ndv.error]
 which determines the error tolerance for "distinct_count". Error tolerance provides a tradeoff between accuracy vs compute cost.

In order to get the correct count, suggest setting {color:#FF0000}hive.stats.ndv.error = 0{color}
 I have tested this on EMR cluster and it indeed improves the accuracy to 100%

Another interesting property, if you are using partitions, is to consider tuning hive.metastore.stats.ndv.tuner to be closer to 1. The default is 0.

> Column statistics give erraneous numDistinct
> --------------------------------------------
>
>                 Key: HIVE-20574
>                 URL: https://issues.apache.org/jira/browse/HIVE-20574
>             Project: Hive
>          Issue Type: Bug
>          Components: Metastore, Statistics
>    Affects Versions: 2.3.2
>         Environment: Amazon EMR (BigTop based) from emr-5.9.0 to emr-5.16.0.
>            Reporter: Mikko Kivistö
>            Priority: Major
>              Labels: Statistics, statsCollection
>
> 1) Download the parquet file to s3/hdfs (e.g. hdfs:///tmp/testi_parquet/) using some tool (aws cli, hdfs command or anything)
>    - S3: s3://www.smartdatahub.io/data/test.parquet
>    - HTTP: [http://www.smartdatahub.io/data/test.parquet]
>    - or the attachmen
> eg. with aws cli, wget/curl/distcp can also be used
> {\{ aws s3 cp s3://www.smartdatahub.io/data/test.parquet . }}
> {\{ hdfs dfs -mkdir hdfs:///tmp/testi_parquet/}}hdfs dfs -put test.parquet }}
> {{ hdfs:///tmp/testi_parquet/test.parquet}}
> 2) Create table default.testi_parquet2 on top of that using the schema provided
> {{CREATE TABLE `default.testi_parquet2`(}}
> {{   `rakennustu` int, }}
> {{   `kohdenimi` string, }}
> {{   `tekstisuun` int, }}
> {{   `tekstikoko` float, }}
> {{   `tekstifont` string, }}
> {{   `buix_bid` int, }}
> {{   `paivitetty` string, }}
> {{   `datanomist` string, }}
> {{   `geom_geojson` string, }}
> {{   `geom` binary, }}
> {{   `extractdate` string)}}
> {{ ROW FORMAT SERDE }}
> {{   'org.apache.hadoop.hive.ql.io.parquet.serde.ParquetHiveSerDe' }}
> {{ STORED AS INPUTFORMAT }}
> {{   'org.apache.hadoop.hive.ql.io.parquet.MapredParquetInputFormat' }}
> {{ OUTPUTFORMAT }}
> {{   'org.apache.hadoop.hive.ql.io.parquet.MapredParquetOutputFormat'}}
> {{ LOCATION}}
> {{   'hdfs:///tmp/testi_parquet/';}}
> {{ \{{ – CHANGE THE LOCATION TO THE PREFIX/DIRECTORY YOU DOWNLOADED THE FILE FROM STEP 1}}}}
> 3) To collect the values showing you the actual reality of the data: Query the distinct count, min and max of column "tekstisuun"
> {\{ SELECT COUNT(DISTINCT tekstisuun), MAX(tekstisuun), MIN(tekstisuun) FROM default.testi_parquet2; }}
> and note them  (min 0, max 0, distinct 1)
>  4) Compute statistics for the table using
> {{ANALYZE TABLE default.testi_parquet2 COMPUTE STATISTICS FOR COLUMNS;}}
> 5) See erroneous statistics entry for numDistincts: Query the statistics by using "
> {{DESCRIBE FORMATTED default.testi_parquet2 tekstisuun}}
> " and note the ERRANEOUS numDistincts value: 2



--
This message was sent by Atlassian JIRA
(v7.6.3#76005)