Hi Lefty,
I took a look at the documentation link and I noticed that it can be improved. For example the paragraph below: “How does Hive distribute the rows across the buckets? In general, the bucket number is determined by the expression hash_function(bucketing_column) mod num_buckets. (There's a '0x7FFFFFFF in there too, but that's not that important). The hash_function depends on the type of the bucketing column. For an int, it's easy, hash_int(i) == i. For example, if user_id were an int, and there were 10 buckets, we would expect all user_id's that end in 0 to be in bucket 1, all user_id's that end in a 1 to be in bucket 2, etc. For other datatypes, it's a little tricky. In particular, the hash of a BIGINT is not the same as the BIGINT. And the hash of a string or a complex datatype will be some number that's derived from the value, but not anything humanly-recognizable. For example, if user_id were a STRING, then the user_id's in bucket 1 would probably not end in 0. In general, distributing rows based on the hash will give you a even distribution in the buckets. So, what can go wrong? As long as you set hive.enforce.bucketing = true, and use the syntax above, the tables should be populated properly. Things can go wrong if the bucketing column type is different during the insert and on read, or if you manually cluster by a value that's different from the table definition.” So in a nutshell num_buckets determines the granularity of hashing and the number of files. So eventually the table will have in total number_partitions x num_buckets files. The example mentions (not shown above) 256 buckets but that is just a number. It also states “For example, …and there were 10 buckets”. This is not standard. In a nutshell bucketing is a method to get data “evenly distributed” over many files. Thus, one should define the number of num_buckets by a power of two -- 2^n, like 2, 4, 8, 16 etc to achieve best results and getting best clustering. I will try to see the upper limits on the number of buckets within a partition and will get back on that. HTH Mich Talebzadeh http://talebzadehmich.wordpress.com Author of the books "A Practitioner’s Guide to Upgrading to Sybase ASE 15", ISBN 978-0-9563693-0-7. co-author "Sybase Transact SQL Guidelines Best Practices", ISBN 978-0-9759693-0-4 Publications due shortly: Creating in-memory Data Grid for Trading Systems with Oracle TimesTen and Coherence Cache Oracle and Sybase, Concepts and Contrasts, ISBN: 978-0-9563693-1-4, volume one out shortly NOTE: The information in this email is proprietary and confidential. This message is for the designated recipient only, if you are not the intended recipient, you should destroy it immediately. Any information in this message shall not be understood as given or endorsed by Peridale Ltd, its subsidiaries or their employees, unless expressly so stated. It is the responsibility of the recipient to ensure that this email is virus free, therefore neither Peridale Ltd, its subsidiaries nor their employees accept any responsibility. From: Lefty Leverenz [mailto:leftylever...@gmail.com] Sent: 17 April 2015 00:06 To: user@hive.apache.org Subject: Re: [Hive 0.13.1] - Explanation/confusion over "Fatal error occurred when node tried to create too many dynamic partitions" on small dataset with dynamic partitions If the number of buckets in a partitioned table has a limit, we need to document it in the wiki. Currently the example <https://cwiki.apache.org/confluence/display/Hive/LanguageManual+DDL+BucketedTables> shows 256 buckets. -- Lefty On Thu, Apr 16, 2015 at 4:35 AM, Daniel Harper <daniel.har...@bbc.co.uk> wrote: As in you can only have 32 buckets (rather than 128 in our case?) With thanks, Daniel Harper Software Engineer, OTG ANT BC5 A5 From: Mich Talebzadeh <m...@peridale.co.uk> Reply-To: "user@hive.apache.org" <user@hive.apache.org> Date: Wednesday, 15 April 2015 16:56 To: "user@hive.apache.org" <user@hive.apache.org> Subject: RE: [Hive 0.13.1] - Explanation/confusion over "Fatal error occurred when node tried to create too many dynamic partitions" on small dataset with dynamic partitions ---------------------------- http://www.bbc.co.uk This e-mail (and any attachments) is confidential and may contain personal views which are not the views of the BBC unless specifically stated. If you have received it in error, please delete it from your system. Do not use, copy or disclose the information in any way nor act in reliance on it and notify the sender immediately. Please note that the BBC monitors e-mails sent or received. Further communication will signify your consent to this. ---------------------
