https://community.cloudera.com/t5/Support-Questions/What-is-col0-in-hive-explain-plan/m-p/191217#M153306 <P>"_col0" is an internal/generated column. If you run a "select count(*) from foo" hive does not have to read any columns from the table, it just needs to count the records. What the explain plan says is that the table scan and select operator first generate empty records from the table. Then the group by counts those and stores that in a generated column "_col0". (There is a second group by because Hive has to aggregate all the results from the different "mappers".)</P><P>If you just count all rows it's odd to run out of memory. What's the actual query and failure you're seeing?</P> Wed, 16 May 2018 01:36:15 GMT ghagleitner 2018-05-16T01:36:15Z