Github user rick-ibm commented on the pull request:
https://github.com/apache/spark/pull/9202#issuecomment-152638711
Thanks for that feedback, Michael. To answer your question:
MA> I haven't looked closely at the implementation, but one high level
question is whether this is breaking the use case where a users gives a
subquery instead of a table name (i.e. dbtable = (SELECT ...).) This is an
important part of the API that we can't break.
Reynold posed the same question at
https://issues.apache.org/jira/browse/SPARK-10857?focusedCommentId=14967827&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14967827
and I replied at
https://issues.apache.org/jira/browse/SPARK-10857?focusedCommentId=14967930&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14967930
Can you give me an example of the syntax you have in mind which might trip
this problem? I tried the following experiment...
val jdbcDF = sqlContext.read.format("jdbc").options(
Map("url" -> "jdbc:derby:/Users/rhillegas/derby/databases/derby1",
"dbtable" -> "select * from app.t")).load()
...but that statement doesn't tickle JdbcDialect.getTableExistsQuery().
Instead, the statement fails because the following ungrammatical query is sent
to the database:
SELECT * FROM select * from app.t WHERE 1=0
That, in turn, is because JDBCRDD.resolveTable() has its own hard-coded
query which probes for the existence of a table. I have logged
https://issues.apache.org/jira/browse/SPARK-11426 to track this issue. The
header comment on JDBCRDD.resolveTable() indicates that the method expects a
table name and not an arbitrary query.
Thanks,
-Rick
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