Thanks for sharing your view. I agree with you it's good for Spark to promote Spark's own CREATE TABLE syntax. The thing is, we still leave Hive CREATE TABLE syntax unchanged - it's being said as "convenience" but I'm not sure I can agree with.
I'll quote my comments in SPARK-31136 here again to make the problem statement be clearer: I think the parser implementation around CREATE TABLE brings ambiguity which is not documented anywhere. It wasn't ambiguous because we forced to specify USE provider if it's not a Hive table. Now it's either default provider or Hive according to which options are provided, which seems to be non-trivial to reason about. (End users would never know, as it's completely from parser rule.) I feel this as the issue of "not breaking old behavior". The parser rule gets pretty much complicated due to support legacy config. Not breaking anything would make us be stuck eventually. https://github.com/apache/spark/blob/master/docs/sql-migration-guide.md Since Spark 3.0, CREATE TABLE without a specific provider will use the value of spark.sql.sources.default as its provider. In Spark version 2.4 and earlier, it was hive. To restore the behavior before Spark 3.0, you can set spark.sql.legacy.createHiveTableByDefault.enabled to true. It's not true if "ROW FORMAT" / "STORED AS" are provided, and we didn't describe anything for this. https://github.com/apache/spark/blob/master/docs/sql-ref-syntax-ddl-create-table-datasource.md CREATE TABLE [ IF NOT EXISTS ] table_identifier [ ( col_name1 col_type1 [ COMMENT col_comment1 ], ... ) ] [USING data_source] [ OPTIONS ( key1=val1, key2=val2, ... ) ] [ PARTITIONED BY ( col_name1, col_name2, ... ) ] [ CLUSTERED BY ( col_name3, col_name4, ... ) [ SORTED BY ( col_name [ ASC | DESC ], ... ) ] INTO num_buckets BUCKETS ] [ LOCATION path ] [ COMMENT table_comment ] [ TBLPROPERTIES ( key1=val1, key2=val2, ... ) ] [ AS select_statement ] https://github.com/apache/spark/blob/master/docs/sql-ref-syntax-ddl-create-table-hiveformat.md CREATE [ EXTERNAL ] TABLE [ IF NOT EXISTS ] table_identifier [ ( col_name1[:] col_type1 [ COMMENT col_comment1 ], ... ) ] [ COMMENT table_comment ] [ PARTITIONED BY ( col_name2[:] col_type2 [ COMMENT col_comment2 ], ... ) | ( col_name1, col_name2, ... ) ] [ ROW FORMAT row_format ] [ STORED AS file_format ] [ LOCATION path ] [ TBLPROPERTIES ( key1=val1, key2=val2, ... ) ] [ AS select_statement ] At least we should describe that parser will try to match the first case (create table ~ using data source), and fail back to second case; even though we describe this it's not intuitive to reason about which rule the DDL query will fall into. As I commented earlier, "ROW FORMAT" and "STORED AS" are the options which make DDL query fall into the second case, but they're described as "optional" so it's hard to catch the gotcha. Furthermore, while we document the syntax as above, in reality we allow "EXTERNAL" in first rule (and throw error), which ends up existing DDL query "CREATE EXTERNAL TABLE ~ LOCATION" be broken. Even we add "USING hive" parser will throw error. It now requires "ROW FORMAT" or "STORED AS". Simply saying, do we really think end users should stop and try to match their query against the parser rules (or orders when we explain in the doc) by themselves to understand which provider the table will leverage? I'm sorry but I think we are making bad assumption on end users which is a serious problem. If we really want to promote Spark's one for CREATE TABLE, then would it really matter to treat Hive CREATE TABLE be "exceptional" one and try to isolate each other? What's the point of providing a legacy config to go back to the old one even we fear about breaking something to make it better or clearer? We do think that table provider is important (hence the change was done), then is it still a trivial problem whether the provider is affected by specifying the "optional" fields? On Wed, Mar 18, 2020 at 4:38 PM Wenchen Fan <cloud0...@gmail.com> wrote: > I think the general guideline is to promote Spark's own CREATE TABLE > syntax instead of the Hive one. Previously these two rules are mutually > exclusive because the native syntax requires the USING clause while the > Hive syntax makes ROW FORMAT or STORED AS clause optional. > > It's a good move to make the USING clause optional, which makes it easier > to write the native CREATE TABLE syntax. Unfortunately, it leads to some > conflicts with the Hive CREATE TABLE syntax, but I don't see a serious > problem here. If a user just writes CREATE TABLE without USING or ROW > FORMAT or STORED AS, does it matter what table we create? Internally the > parser rules conflict and we pick the native syntax depending on the rule > order. But the user-facing behavior looks fine. > > CREATE EXTERNAL TABLE is a problem as it works in 2.4 but not in 3.0. > Shall we simply remove EXTERNAL from the native CREATE TABLE syntax? Then > CREATE EXTERNAL TABLE creates Hive table like 2.4. > > On Mon, Mar 16, 2020 at 10:55 AM Jungtaek Lim < > kabhwan.opensou...@gmail.com> wrote: > >> Hi devs, >> >> I'd like to initiate discussion and hear the voices for resolving >> ambiguous parser rule between two "create table"s being brought by >> SPARK-30098 [1]. >> >> Previously, "create table" parser rules were clearly distinguished via >> "USING provider", which was very intuitive and deterministic. Say, DDL >> query creates "Hive" table unless "USING provider" is specified, >> (Please refer the parser rule in branch-2.4 [2]) >> >> After SPARK-30098, "create table" parser rules became ambiguous (please >> refer the parser rule in branch-3.0 [3]) - the factors differentiating two >> rules are only "ROW FORMAT" and "STORED AS" which are all defined as >> "optional". Now it relies on the "order" of parser rule which end users >> would have no idea to reason about, and very unintuitive. >> >> Furthermore, undocumented rule of EXTERNAL (added in the first rule to >> provide better message) brought more confusion (I've described the broken >> existing query via SPARK-30436 [4]). >> >> Personally I'd like to see two rules mutually exclusive, instead of >> trying to document the difference and talk end users to be careful about >> their query. I'm seeing two ways to make rules be mutually exclusive: >> >> 1. Add some identifier in create Hive table rule, like `CREATE ... "HIVE" >> TABLE ...`. >> >> pros. This is the simplest way to distinguish between two rules. >> cons. This would lead end users to change their query if they intend to >> create Hive table. (Given we will also provide legacy option I'm feeling >> this is acceptable.) >> >> 2. Define "ROW FORMAT" or "STORED AS" as mandatory one. >> >> pros. Less invasive for existing queries. >> cons. Less intuitive, because they have been optional and now become >> mandatory to fall into the second rule. >> >> Would like to hear everyone's voices; better ideas are welcome! >> >> Thanks, >> Jungtaek Lim (HeartSaVioR) >> >> 1. SPARK-30098 Use default datasource as provider for CREATE TABLE syntax >> https://issues.apache.org/jira/browse/SPARK-30098 >> 2. >> https://github.com/apache/spark/blob/branch-2.4/sql/catalyst/src/main/antlr4/org/apache/spark/sql/catalyst/parser/SqlBase.g4 >> 3. >> https://github.com/apache/spark/blob/branch-3.0/sql/catalyst/src/main/antlr4/org/apache/spark/sql/catalyst/parser/SqlBase.g4 >> 4. https://issues.apache.org/jira/browse/SPARK-30436 >> >>
