@Test
  public void testSkipAcquireLocksForExplain() throws Exception {
    dropTable(new String[] {"tab_acid"});

    driver.run("create table if not exists tab_acid (a int) partitioned by (p string) " +
      "stored as orc TBLPROPERTIES ('transactional'='true')");
    driver.run("insert into tab_acid values(1,'foo'),(3,'bar')");

    driver.compileAndRespond("explain update tab_acid set a = a+2 where a > 2", true);
    driver.lockAndRespond();

    List<ShowLocksResponseElement> locks = getLocks();
    Assert.assertEquals("Unexpected lock count", 0, locks.size());
  }
T7 is a table with 2 partitions
1. run select from T7
2. run drop partition from T7
concurrently with 1 starting first so that 2 blocks
3. start another concurrent select on T7 - it should block behind waiting X (from drop) - LM should be fair
4. finish #1 so that drop unblocks
5. rollback the drop to release its X lock
6. # should unblock
ql/src/test/org/apache/hadoop/hive/ql/lockmgr/TestDbTxnManager2.java