如果你的查询不断，那得做预连接。而且针对的好像只是select(我的测试)。客户端的tnsnames,ora得做相应的配置。

failover与load balance是不能并存的。(我的测试)

给你一个各种各样failover配置的例子：
# TNSNAMES.ORA Network Configuration File: e:\stuff\tnsnames.ora
# Generated by Oracle configuration tools.

XXN1.world =
  (DESCRIPTION =
    (ADDRESS_LIST =
       (ADDRESS =(PROTOCOL = TCP)(Host = git-opssun1)(Port = 1521))
     )
    (CONNECT_DATA = (SID = XXN1))
   )

XXN2.world =
  (DESCRIPTION =
    (ADDRESS_LIST =
       (ADDRESS =(PROTOCOL = TCP)(Host = git-opssun2)(Port = 1521))
     )
    (CONNECT_DATA = (SID = XXN2))
   )
                                         

# Try each address, in order, until one succeeds

ALIAS1.WORLD =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = git-opssun1)(PORT = 1521))
      (ADDRESS = (PROTOCOL = TCP)(HOST = git-opssun2)(PORT = 1521))
    )
    (CONNECT_DATA =
      (SERVICE_NAME = XX.git-opssun.oraclecorp.com)
      (failover_mode=
        (type=session)
        (method=basic)
        (retries=20)
        (delay=15))
    )
  )

# Use each address in order until destination reached

ALIAS2.WORLD =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = git-opssun1)(PORT = 1521))
      (ADDRESS = (PROTOCOL = TCP)(HOST = git-opssun2)(PORT = 1521))
      (LOAD_BALANCE = yes)
      (FAILOVER = true)
    )
    (CONNECT_DATA =
      (SERVICE_NAME = XX.git-opssun.oraclecorp.com)
      (failover_mode=
        (type=session)
        (method=basic)
        (retries=20)
        (delay=15))
    )
  )

# Try each address, randomly, until one succeeds

ALIAS3.WORLD =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = git-opssun1)(PORT = 1521))
      (ADDRESS = (PROTOCOL = TCP)(HOST = git-opssun2)(PORT = 1521))
      (LOAD_BALANCE = yes)
    )
    (CONNECT_DATA =
      (SERVICE_NAME = XX.git-opssun.oraclecorp.com)
    )
  )

# Try one address selected at random

ALIAS4.WORLD =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = git-opssun1)(PORT = 1521))
      (ADDRESS = (PROTOCOL = TCP)(HOST = git-opssun2)(PORT = 1521))
      (LOAD_BALANCE = yes)
      (FAILOVER = true)
    )
    (CONNECT_DATA =
      (SERVICE_NAME = XX.git-opssun.oraclecorp.com)
      (failover_mode=
        (type=select)
        (method=basic)
        (retries=20)
        (delay=15))
    )
  )

# Use only the first address

ALIAS5.WORLD =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = git-opssun1)(PORT = 1521))
      (ADDRESS = (PROTOCOL = TCP)(HOST = git-opssun2)(PORT = 1521))
      (FAILOVER = false)
    )
    (CONNECT_DATA =
      (SERVICE_NAME = XX.git-opssun.oraclecorp.com)
      (failover_mode=
        (type=select)
        (method=basic)
        (retries=20)
        (delay=15))
    )
  )

#alias using backup syntax

ALIAS6.WORLD =
  (DESCRIPTION =
    (load_balance=off)
    (failover=true)
      (ADDRESS = (PROTOCOL = TCP)(HOST = git-opssun1)(PORT = 1521))
    (CONNECT_DATA =
      (SERVICE_NAME = XX.git-opssun.oraclecorp.com)
     (failover_mode=
       (backup=ALIAS7.world)
       (type=select)
       (method=PRECONNECT)
       (retries=20)
       (delay=15)
      )
    )
   )

ALIAS7.WORLD =
  (DESCRIPTION =
    (load_balance=off)
    (failover=true)
      (ADDRESS = (PROTOCOL = TCP)(HOST = git-opssun2)(PORT = 1521))
    (CONNECT_DATA =
      (SERVICE_NAME = XX.git-opssun.oraclecorp.com)
     (failover_mode=
       (backup=ALIAS6.world)
       (type=select)
       (method=PRECONNECT)
       (retries=20)
       (delay=15)
      )
    )
   )