dtenedor opened a new pull request #35690:
URL: https://github.com/apache/spark/pull/35690
### What changes were proposed in this pull request?
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Implement parser changes needed to support for DEFAULT column values as
tracked in https://issues.apache.org/jira/browse/SPARK-38334.
Note that these are the parser changes only. Analysis support will take
place in a following PR.
Background: in the future, CREATE TABLE and ALTER TABLE invocations will
support setting column default values for later operations. Following INSERT,
UPDATE, MERGE statements may then reference the value using the DEFAULT keyword
as needed.
Examples:
```sql
CREATE TABLE T(a INT, b INT NOT NULL);
-- The default default is NULL
INSERT INTO T VALUES (DEFAULT, 0);
INSERT INTO T(b) VALUES (1);
SELECT * FROM T;
(NULL, 0)
(NULL, 1)
-- Adding a default to a table with rows, sets the values for the
-- existing rows (exist default) and new rows (current default).
ALTER TABLE T ADD COLUMN c INT DEFAULT 5;
INSERT INTO T VALUES (1, 2, DEFAULT);
SELECT * FROM T;
(NULL, 0, 5)
(NULL, 1, 5)
(1, 2, 5)
```
### Why are the changes needed?
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This new API helps users run DDL and DML statements to add new values to
tables and scan existing values from tables more easily.
### Does this PR introduce _any_ user-facing change?
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the documentation fix.
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No
### How was this patch tested?
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Added unit test coverage in DDLParserSuite.scala
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