起由：在本机启动两个数据库， orcl和HIS， 在服务端连接没有问题。采用客户端模式连接时，在PL/SQL连接数据库时，经常会报以下两个错误。
ORA-12154: TNS:could not resolve the connect identifierspecified
ORA-12523: TNS:listener could not find instance appropriate for the client connection
在后台，用SQLPlus命令connect 连接，显示CONNECTED。说明服务启动正常，只是PL/SQL连接不上数据库。
如果关掉一个数据库，只启动一个数据库时，连接就是好的。如果两个数据库启动后，再连接，就会继续报上面的错误，一个也连接不上。
客户端文件：tnsnames.ora，内容如下：
HIS =
(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = 127.0.0.1)(PORT = 1521))
)
(CONNECT_DATA =
(SERVICE_NAME = his)
)
)
ORCL =
(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = 127.0.0.1)(PORT = 1521))
)
(CONNECT_DATA =
(SERVICE_NAME = ORCL)
)
)
将HOST = 127.0.0.1改成HOST = LOCALHOST，如下：
HIS =
(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = LOCALHOST)(PORT = 1521))
)
(CONNECT_DATA =
(SERVICE_NAME = his)
)
)
ORCL =
(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = LOCALHOST)(PORT = 1521))
)
(CONNECT_DATA =
(SERVICE_NAME = ORCL)
)
)
关闭PL/SQL, 重新再连接，OK，一切正常。
原来一直认为TCP连接中，127.0.0.1就是LOCALHOST，理解有误，尽管当前浏览器上访问时，127.0.0.1和LOCALHOST访问的是同一个地址。