{//最简单、原始的、可能的查询计划
public Plan createPlan(QueryData data, Transaction tx)
{
//第一步：为每一个出现的表或者视图定义创建一个plan
List <Plan> plans = new List <Plan>();
foreach (string tblname in data.tables())
{
string viewdef = SimpleDB.mdMgr().getViewDef(tblname, tx);
if (viewdef != null)
{
plans.Add(SimpleDB.planner().createQueryPlan(viewdef, tx));
}
else
{
plans.Add(new TablePlan(tblname, tx));
}
}
//第二步：创建所有表计划的叉积运算
Plan p = plans[0];
plans.RemoveAt(0);
foreach (Plan nextplan in plans)
{
p = new ProductPlan(p, nextplan);
}
//第三步：为predicate添加一个选择运算selection
p = new SelectPlan(p, data.pred());
//第四步：做字段名称上的投影运算
p = new ProjectPlan(p, data.fields());
return(p);
}
public int executeDelete(DeleteData data, Transaction tx)
{
Plan p = new TablePlan(data.tableName(), tx);
p = new SelectPlan(p, data.pred());
UpdateScan us = (UpdateScan)p.open();
int count = 0;
while (us.next())
{
us.delete();
count++;
}
us.close();
return(count);
}
public int executeModify(ModifyData data, Transaction tx)
{
Plan p = new TablePlan(data.tableName(), tx);
p = new SelectPlan(p, data.pred());
UpdateScan us = (UpdateScan)p.open();
int count = 0;
while (us.next())
{
Constant val = data.newValue().evaluate(us); //返回此处的表达式的常量值
us.setVal(data.targetField(), val); //设置要更改字段处的值
count++;
}
us.close();
return(count);
}