Hi Ryan, That's a good point. Since in this case Spark is just a channel to pass user's action to the data source, we should think of what actions the data source supports.
Following this direction, it makes more sense to delegate everything to data sources. As the first step, maybe we should not add DDL commands to change schema of data source, but just use the capability API to let data source decide what to do when input schema doesn't match the table schema during writing. Users can use native client of data source to change schema. On Fri, Dec 21, 2018 at 8:03 AM Ryan Blue <rb...@netflix.com> wrote: > I think it is good to know that not all sources support default values. > That makes me think that we should delegate this behavior to the source and > have a way for sources to signal that they accept default values in DDL (a > capability) and assume that they do not in most cases. > > On Thu, Dec 20, 2018 at 1:32 PM Russell Spitzer <russell.spit...@gmail.com> > wrote: > >> I guess my question is why is this a Spark level behavior? Say the user >> has an underlying source where they have a different behavior at the source >> level. In Spark they set a new default behavior and it's added to the >> catalogue, is the Source expected to propagate this? Or does the user have >> to be aware that their own Source settings may be different for a client >> connecting via Spark or via a native driver. >> >> For example say i'm using C* (sorry but obviously I'm always thinking >> about C*), and I add a new column to the database. When i connect to the >> database with a non-spark application I expect to be able to insert to the >> table given that I satisfy the required columns. In Spark someone sets the >> columns as having a default value (there is no such feature in C*), now >> depending on how I connect to the source I have two different behaviors. If >> I insert from the native app I get empty cells, if I insert from spark i >> get a default value inserted. That sounds more confusing to an end-user to >> than having a consistent behavior between native clients and Spark clients. >> This is why I asked if the goal was to just have a common "Spark" behavior >> because I don't think it makes sense if you consider multiple interaction >> points for a source. >> >> On Wed, Dec 19, 2018 at 9:28 PM Wenchen Fan <cloud0...@gmail.com> wrote: >> >>> So you agree with my proposal that we should follow RDBMS/SQL standard >>> regarding the behavior? >>> >>> > pass the default through to the underlying data source >>> >>> This is one way to implement the behavior. >>> >>> On Thu, Dec 20, 2018 at 11:12 AM Ryan Blue <rb...@netflix.com> wrote: >>> >>>> I don't think we have to change the syntax. Isn't the right thing (for >>>> option 1) to pass the default through to the underlying data source? >>>> Sources that don't support defaults would throw an exception. >>>> >>>> On Wed, Dec 19, 2018 at 6:29 PM Wenchen Fan <cloud0...@gmail.com> >>>> wrote: >>>> >>>>> The standard ADD COLUMN SQL syntax is: ALTER TABLE table_name ADD >>>>> COLUMN column_name datatype [DEFAULT value]; >>>>> >>>>> If the DEFAULT statement is not specified, then the default value is >>>>> null. If we are going to change the behavior and say the default value is >>>>> decided by the underlying data source, we should use a new SQL syntax(I >>>>> don't have a proposal in mind), instead of reusing the existing syntax, to >>>>> be SQL compatible. >>>>> >>>>> Personally I don't like re-invent wheels. It's better to just >>>>> implement the SQL standard ADD COLUMN command, which means the default >>>>> value is decided by the end-users. >>>>> >>>>> On Thu, Dec 20, 2018 at 12:43 AM Ryan Blue <rb...@netflix.com> wrote: >>>>> >>>>>> Wenchen, can you give more detail about the different ADD COLUMN >>>>>> syntax? That sounds confusing to end users to me. >>>>>> >>>>>> On Wed, Dec 19, 2018 at 7:15 AM Wenchen Fan <cloud0...@gmail.com> >>>>>> wrote: >>>>>> >>>>>>> Note that the design we make here will affect both data source >>>>>>> developers and end-users. It's better to provide reliable behaviors to >>>>>>> end-users, instead of asking them to read the spec of the data source >>>>>>> and >>>>>>> know which value will be used for missing columns, when they write data. >>>>>>> >>>>>>> If we do want to go with the "data source decides default value" >>>>>>> approach, we should create a new SQL syntax for ADD COLUMN, as its >>>>>>> behavior >>>>>>> is different from the SQL standard ADD COLUMN command. >>>>>>> >>>>>>> On Wed, Dec 19, 2018 at 10:58 PM Russell Spitzer < >>>>>>> russell.spit...@gmail.com> wrote: >>>>>>> >>>>>>>> I'm not sure why 1) wouldn't be fine. I'm guessing the reason we >>>>>>>> want 2 is for a unified way of dealing with missing columns? I feel >>>>>>>> like >>>>>>>> that probably should be left up to the underlying datasource >>>>>>>> implementation. For example if you have missing columns with a >>>>>>>> database the >>>>>>>> Datasource can choose a value based on the Database's metadata if such >>>>>>>> a >>>>>>>> thing exists, I don't think Spark should really have a this level of >>>>>>>> detail >>>>>>>> but I've also missed out on all of these meetings (sorry it's family >>>>>>>> dinner >>>>>>>> time :) ) so I may be missing something. >>>>>>>> >>>>>>>> So my tldr is, Let a datasource report whether or not missing >>>>>>>> columns are OK and let the Datasource deal with the missing data based >>>>>>>> on >>>>>>>> it's underlying storage. >>>>>>>> >>>>>>>> On Wed, Dec 19, 2018 at 8:23 AM Wenchen Fan <cloud0...@gmail.com> >>>>>>>> wrote: >>>>>>>> >>>>>>>>> I agree that we should not rewrite existing parquet files when a >>>>>>>>> new column is added, but we should also try out best to make the >>>>>>>>> behavior >>>>>>>>> same as RDBMS/SQL standard. >>>>>>>>> >>>>>>>>> 1. it should be the user who decides the default value of a >>>>>>>>> column, by CREATE TABLE, or ALTER TABLE ADD COLUMN, or ALTER TABLE >>>>>>>>> ALTER >>>>>>>>> COLUMN. >>>>>>>>> 2. When adding a new column, the default value should be effective >>>>>>>>> for all the existing data, and newly written data. >>>>>>>>> 3. When altering an existing column and change the default value, >>>>>>>>> it should be effective for newly written data only. >>>>>>>>> >>>>>>>>> A possible implementation: >>>>>>>>> 1. a columnn has 2 default values: the initial one and the latest >>>>>>>>> one. >>>>>>>>> 2. when adding a column with a default value, set both the initial >>>>>>>>> one and the latest one to this value. But do not update existing data. >>>>>>>>> 3. when reading data, fill the missing column with the initial >>>>>>>>> default value >>>>>>>>> 4. when writing data, fill the missing column with the latest >>>>>>>>> default value >>>>>>>>> 5. when altering a column to change its default value, only update >>>>>>>>> the latest default value. >>>>>>>>> >>>>>>>>> This works because: >>>>>>>>> 1. new files will be written with the latest default value, >>>>>>>>> nothing we need to worry about at read time. >>>>>>>>> 2. old files will be read with the initial default value, which >>>>>>>>> returns expected result. >>>>>>>>> >>>>>>>>> On Wed, Dec 19, 2018 at 8:39 AM Ryan Blue >>>>>>>>> <rb...@netflix.com.invalid> wrote: >>>>>>>>> >>>>>>>>>> Hi everyone, >>>>>>>>>> >>>>>>>>>> This thread is a follow-up to a discussion that we started in the >>>>>>>>>> DSv2 community sync last week. >>>>>>>>>> >>>>>>>>>> The problem I’m trying to solve is that the format I’m using DSv2 >>>>>>>>>> to integrate supports schema evolution. Specifically, adding a new >>>>>>>>>> optional >>>>>>>>>> column so that rows without that column get a default value (null for >>>>>>>>>> Iceberg). The current validation rule for an append in DSv2 fails a >>>>>>>>>> write >>>>>>>>>> if it is missing a column, so adding a column to an existing table >>>>>>>>>> will >>>>>>>>>> cause currently-scheduled jobs that insert data to start failing. >>>>>>>>>> Clearly, >>>>>>>>>> schema evolution shouldn't break existing jobs that produce valid >>>>>>>>>> data. >>>>>>>>>> >>>>>>>>>> To fix this problem, I suggested option 1: adding a way for Spark >>>>>>>>>> to check whether to fail when an optional column is missing. Other >>>>>>>>>> contributors in the sync thought that Spark should go with option 2: >>>>>>>>>> Spark’s schema should have defaults and Spark should handle filling >>>>>>>>>> in >>>>>>>>>> defaults the same way across all sources, like other databases. >>>>>>>>>> >>>>>>>>>> I think we agree that option 2 would be ideal. The problem is >>>>>>>>>> that it is very hard to implement. >>>>>>>>>> >>>>>>>>>> A source might manage data stored in millions of immutable >>>>>>>>>> Parquet files, so adding a default value isn’t possible. Spark would >>>>>>>>>> need >>>>>>>>>> to fill in defaults for files written before the column was added at >>>>>>>>>> read >>>>>>>>>> time (it could fill in defaults in new files at write time). Filling >>>>>>>>>> in >>>>>>>>>> defaults at read time would require Spark to fill in defaults for >>>>>>>>>> only some >>>>>>>>>> of the files in a scan, so Spark would need different handling for >>>>>>>>>> each >>>>>>>>>> task depending on the schema of that task. Tasks would also be >>>>>>>>>> required to >>>>>>>>>> produce a consistent schema, so a file without the new column >>>>>>>>>> couldn’t be >>>>>>>>>> combined into a task with a file that has the new column. This adds >>>>>>>>>> quite a >>>>>>>>>> bit of complexity. >>>>>>>>>> >>>>>>>>>> Other sources may not need Spark to fill in the default at all. A >>>>>>>>>> JDBC source would be capable of filling in the default values >>>>>>>>>> itself, so >>>>>>>>>> Spark would need some way to communicate the default to that source. >>>>>>>>>> If the >>>>>>>>>> source had a different policy for default values (write time instead >>>>>>>>>> of >>>>>>>>>> read time, for example) then behavior could still be inconsistent. >>>>>>>>>> >>>>>>>>>> I think that this complexity probably isn’t worth consistency in >>>>>>>>>> default values across sources, if that is even achievable. >>>>>>>>>> >>>>>>>>>> In the sync we thought it was a good idea to send this out to the >>>>>>>>>> larger group to discuss. Please reply with comments! >>>>>>>>>> >>>>>>>>>> rb >>>>>>>>>> -- >>>>>>>>>> Ryan Blue >>>>>>>>>> Software Engineer >>>>>>>>>> Netflix >>>>>>>>>> >>>>>>>>> >>>>>> >>>>>> -- >>>>>> Ryan Blue >>>>>> Software Engineer >>>>>> Netflix >>>>>> >>>>> >>>> >>>> -- >>>> Ryan Blue >>>> Software Engineer >>>> Netflix >>>> >>> > > -- > Ryan Blue > Software Engineer > Netflix >
