Mingli-Rui opened a new pull request, #38452:
URL: https://github.com/apache/spark/pull/38452
### What changes were proposed in this pull request?
This PR changed to resolve the JDBC Relation schema with
PreparedStatement.executeQuery() instead of PreparedStatement.getMetaData().
The main changes are:
1. Add a new function: getQueryOutputSchema(...) to JdbcDialect. By default
it will use getMetaData() to resolve schema.
2. For 3 database dialect: H2Dialect, MsSqlServerDialect and MySqlDialect,
override this function to use executeQuery() to resolve the schema because they
doesn't support getMetaData() well.
3. Add 2 utility functions to JdbcUtils for the 2 ways to resolving schema.
### Why are the changes needed?
1. getMetaData() is better performance compared with executeQuery() because
getMetaData() only needs to compile the Query, but executeQuery() needs
to execute the query.
2. Move the schema resolution to JdbcDialect, make it possible to resolve
schema differently for each database.
In the future, spark can support some non-SELECT query such as CALL
PROC, etc.
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
Tested with existing test
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