I added this functionality <https://github.com/apache/impala/commit/49610e2cfa40aa10b626c5ae41d7f0d99d7cabc5> 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 <edlinuxg...@gmail.com> 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 <t...@cloudera.com.invalid> > 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 > > >
