Github user JoshRosen commented on a diff in the pull request:
https://github.com/apache/spark/pull/15196#discussion_r80100267
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/jdbc/JDBCRDD.scala
---
@@ -58,11 +58,11 @@ object JDBCRDD extends Logging {
val dialect = JdbcDialects.get(url)
val conn: Connection = JdbcUtils.createConnectionFactory(url,
properties)()
try {
- val statement = conn.prepareStatement(s"SELECT * FROM $table WHERE
1=0")
+ val statement =
conn.prepareStatement(dialect.getTableExistsQuery(table))
--- End diff --
I think that this might break schema inference for certain dialects because
it's possible that the result schema of `getTableExistsQuery(table)` will be
different than `select * from $table)`. Instead, we might need to define a new
method in `JdbcDialect` to handle this schema-inference case. I think the goal
of the `WHERE 1=0` was to coerece the remote JDBC database into telling us what
the schema of the query's result set would be without actually returning any
rows or executing the query (in theory we could use `LIMIT 1` but that could be
expensive for, say, aggregate queries).
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