ORC file has the following stats levels for storage indexes
1. ORC File itself 2. Multiple stripes (chunks) within the ORC file 3. Multiple row groups (row batches) within each stripe Assuming that the underlying table has stats updated, count will be stored for each column So when we do something like below: select count(1) from orctest you can see stats collected if you do show create table orctest; TBLPROPERTIES ( | | 'COLUMN_STATS_ACCURATE'='true', | | 'numFiles'='31', | | *'numRows'='250000'*, | File statistics, Stripe statistics and row group statistics are kept. So ORC table will rely on those if needed HTH Dr Mich Talebzadeh LinkedIn * https://www.linkedin.com/profile/view?id=AAEAAAAWh2gBxianrbJd6zP6AcPCCdOABUrV8Pw <https://www.linkedin.com/profile/view?id=AAEAAAAWh2gBxianrbJd6zP6AcPCCdOABUrV8Pw>* http://talebzadehmich.wordpress.com On 22 March 2016 at 07:14, Amey Barve <ameybarv...@gmail.com> wrote: > select count(*) from table; > > How does hive evaluate count(*) on a table? > > Does it return count by actually querying table, or directly return count > by consulting some statistics locally. > > For Hive's Text format it takes few seconds while Hive's Orc format takes > fraction of seconds. > > Regards, > Amey >
