plsql怎么连接本地安装的oracle

 服务器端预处理语句使用服务器端资源来存储语句的执行计划。 阈值提供了一种启发式方法，可以“准备”实际使用的语句。 “经常”的定义默认为5。 
 
 请注意，服务器端预处理语句可能导致执行计划不佳，因为它们不基于准备期间传递的参数。 如果传递给预准备语句的参数对特定索引具有不同的选择性（例如），则预准备语句的常规查询计划可能不是最理想的。 作为另一个示例，如果您遇到查询执行远远大于创建解释计划的成本的情况，并且由于缺少绑定参数而未正确设置解释计划，则最好不要使用服务器端预备语句。 
 
 当驱动程序达到阈值时，它将按如下方式准备语句： 
 
    if (!oneShot)
    {
        // Generate a statement name to use.
        statementName = "S_" + (nextUniqueID++);

        // And prepare the new statement.
        // NB: Must clone the OID array, as it's a direct reference to
        // the SimpleParameterList's internal array that might be modified
        // under us.
        query.setStatementName(statementName);
        query.setStatementTypes((int[])typeOIDs.clone());
    }
 
 语句名称作为有线协议的一部分发送，它告诉Postgres准备服务器端。 

Server side prepared statements consume server side resources to store the execution plan for the statement. The threshold provides a heuristic that causes statements that are actually used "often" to be prepared. The definition of "often" defaults to 5.
 
Note that server side prepared statements can cause poor execution plans because they are not based on the parameters passed during the prepare. If the parameters passed to a prepared statement have a different selectivity on a particular index (for example), then the general query plan of the prepared statement may be suboptimal. As another example, if you have a situation where the execution of the query is much greater than the cost to create an explain plan, and the explain plan isn't properly set due to lack of bind parameters, you may be better off not using server side prepared statements.
 
When the driver reaches the threshold, it will prepare the statement as follows:
 
    if (!oneShot)
    {
        // Generate a statement name to use.
        statementName = "S_" + (nextUniqueID++);

        // And prepare the new statement.
        // NB: Must clone the OID array, as it's a direct reference to
        // the SimpleParameterList's internal array that might be modified
        // under us.
        query.setStatementName(statementName);
        query.setStatementTypes((int[])typeOIDs.clone());
    }
 
The statement name is sent as part of the wire protocol, which tells Postgres to prepare it server side.