Agreed. On Wed, Jul 11, 2018 at 8:55 PM Todd Lipcon <[email protected]> wrote:
> Your commit message there makes sense, Bharath -- we should set > 'avroSchema' in the descriptor in case any referenced partition is avro, > because the scanner needs that info. However, we don't need to also > override the table-level schema. So, I think we can preserve the fix that > you made while also making the behavior less surprising. > > -Todd > > On Wed, Jul 11, 2018 at 8:21 PM, Bharath Vissapragada < > [email protected]> wrote: > > > I added this functionality > > <https://github.com/apache/impala/commit/49610e2cfa40aa10b626c5ae41d7f0 > > d99d7cabc5> > > where adding an Avro partition in a mixed partition table resets the > table > > level schema. While I don't exactly remember why we chose this path, I do > > recall that we debated quite a bit about Avro schema evolution causing > > schema inconsistencies across partitions. AFAICT there is no specific > > reason Impala chose to different from Hive. Now that I see your email, > > Hive's behavior makes more sense to me, especially in the context of lazy > > loading of metadata. > > > > Also, agree with Edward that the whole mixed partitions + Avro schema > > evolution is a mess and I doubt if any serious user relies on a specific > > behavior. > > > > On Wed, Jul 11, 2018 at 7:48 PM Edward Capriolo <[email protected]> > > wrote: > > > > > I know that Hive can deal with schema being different per partition, > but > > I > > > really hesitate to understand why someone would want to do this. If > > someone > > > asked me to support a mixed avro/parquet table I would suggest they > > create > > > a view. If they kept insisting I would reply "Well it is your funeral." > > > > > > On Wed, Jul 11, 2018 at 7:51 PM, Todd Lipcon <[email protected] > > > > > wrote: > > > > > > > Hey folks, > > > > > > > > I'm trying to understand the current behavior of tables that contain > > > > partitions of mixed format, specifically when one or more partitions > is > > > > stored as Avro. Impala seems to be doing a number of things which I > > find > > > > surprising, and I'm not sure if they are intentional or should be > > > > considered bugs. > > > > > > > > *Surprise 1*: the _presence_ of an Avro-formatted partition can > change > > > the > > > > table schema > > > > https://gist.github.com/74bdef8a69b558763e4453ac21313649 > > > > > > > > - create a table that is Parquet-formatted, but with an > > 'avro.schema.url' > > > > property > > > > - the Avro schema is ignored, and we see whatever schema we specified > > > > (*makes > > > > sense, because the table is Parquet)* > > > > - add an partition > > > > - set the new partition's format to Avro > > > > - refresh the table > > > > - the schema for the table now reflects the Avro schema, because it > has > > > at > > > > least one Avro partition > > > > > > > > *Surprise 2*: the above is inconsistent with Hive and Spark > > > > > > > > Hive seems to still reflect the table-level defined schema, and > ignore > > > the > > > > avro.schema.url property in this mixed scenario. That is to say, with > > the > > > > state set up by the above, we have the following behavior: > > > > > > > > Impala: > > > > - uses the external avro schema for all table-level info, SELECT *, > > etc. > > > > - "compute stats" detects the inconsistency and tells the user to > > > recreate > > > > the table. > > > > - if some existing partitions (eg in Parquet) aren't compatible with > > that > > > > avro schema, errors result from the backend that there are missing > > > columns > > > > in the Parquet data files > > > > > > > > Hive: > > > > - uses the table-level schema defined in the HMS for describe, etc > > > > - queries like 'select *' again use the table-level HMS schema. The > > > > underlying reader that reads the Avro partition seems to use the > > defined > > > > external Avro schema, resulting in nulls for missing columns. > > > > - computing stats (analyze table mixedtable partition (y=1) compute > > stats > > > > for columns) seems to end up only recording stats against the column > > > > defined in the table-level Schema. > > > > > > > > Spark: > > > > - DESCRIBE TABLE shows the table-level info > > > > - select * fails, because apparently Spark doesn't support > multi-format > > > > tables at all (it tries to read the avro files as a parquet file) > > > > > > > > > > > > It seems to me that Hive's behavior is a bit better.* I'd like to > > propose > > > > we treat this as a bug and move to the following behavior:* > > > > > > > > - if a table's properties indicate it's an avro table, parse and > adopt > > > the > > > > external avro schema as the table schema > > > > - if a table's properties indicate it's _not_ an avro table, but > there > > is > > > > an external avro schema defined in the table properties, then parse > the > > > > avro schema and include it in the TableDescriptor (for use by avro > > > > partitions) but do not adopt it as the table schema. > > > > > > > > The added benefit of the above proposal (and the reason why I started > > > > looking into this in the first place) is that, in order to service a > > > simple > > > > query like DESCRIBE, our current behavior requires all partition > > metadata > > > > to be loaded to know whether there is any avro-formatted partition. > > With > > > > the proposed new behavior, we can avoid looking at all partitions. > This > > > is > > > > important for any metadata design which supports fine-grained loading > > of > > > > metadata to the coordinator. > > > > > > > > -Todd > > > > -- > > > > Todd Lipcon > > > > Software Engineer, Cloudera > > > > > > > > > > > > > -- > Todd Lipcon > Software Engineer, Cloudera >
