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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