Question

我正在开发一个界面来显示WinForm应用程序中DBF文件的数据。我开始使用OdbcConnection。尽管它有效，但由于Visual FoxPro驱动程序的某些限制（不支持子查询），我开启了使用OLEDB（VFPOLEDB）。现在我能够运行复杂的查询，但是出现了必须解决的新困难。问题是这些查询太慢了。比预期慢100倍。

以下是演示代码。有一个DBF表'PROD'。索引字段PRICE_N用于查询的Where子句中。该表位于运行应用程序的同一台PC上。正如您所看到的，通过ODBC（Microsoft Visual FoxPro驱动程序）和OLEDB（VFPOLEDB）运行查询所花费的时间变化很大。

            TimeSpan timeSpanODBC;
        DateTime timeODBC = DateTime.Now;

        OdbcConnection odbcConnection = new OdbcConnection(@"Driver={Microsoft Visual FoxPro Driver};SourceType=DBF;SourceDB=C:\Users\Vakshul\Documents\dbfs;Exclusive=No;Collate=Machine;NULL=NO;DELETED=NO;BACKGROUNDFETCH=NO;");
        odbcConnection.Open();
        OdbcCommand odbcCommand = new OdbcCommand("SELECT utk_ved FROM prod WHERE (price_n='641857')", odbcConnection);
        odbcCommand.ExecuteScalar();
        timeSpanODBC = DateTime.Now - timeODBC;
        double timeOdbcEqual = timeSpanODBC.TotalMilliseconds;
        System.Console.WriteLine("Time spent via ODBC(milliseconds) using '=' to compare - {0}", timeOdbcEqual.ToString());


        timeODBC = DateTime.Now;

        odbcCommand = new OdbcCommand("SELECT utk_ved FROM prod WHERE (price_n like'641857')", odbcConnection);
        odbcCommand.ExecuteScalar();
        timeSpanODBC = DateTime.Now - timeODBC;
        double timeOdbcLike = timeSpanODBC.TotalMilliseconds;
        System.Console.WriteLine("Time spent via ODBC(milliseconds) using 'Like' to compare - {0}", timeOdbcLike.ToString());

        TimeSpan timeSpanOLEDB;
        DateTime timeOLEDB = DateTime.Now;

        OleDbConnection oleDbCon = new OleDbConnection(@"Provider=VFPOLEDB.1;Data Source=C:\Users\Vakshul\Documents\dbfs;Collating Sequence=MACHINE;Mode=Read");
        oleDbCon.Open();
        OleDbCommand oleDbcommand = new OleDbCommand("SELECT utk_ved FROM prod WHERE (price_n = '641857')", oleDbCon);
        oleDbcommand.ExecuteScalar();
        timeSpanOLEDB = DateTime.Now - timeOLEDB;
        double timeOLEDBEqual = timeSpanOLEDB.TotalMilliseconds;
        System.Console.WriteLine("Time spent via OLEDB(milliseconds) using '=' to compare - {0}", timeOLEDBEqual.ToString());

        timeOLEDB = DateTime.Now;

        oleDbcommand = new OleDbCommand("SELECT utk_ved FROM prod WHERE (price_n like '641857')", oleDbCon);
        oleDbcommand.ExecuteScalar();
        timeSpanOLEDB = DateTime.Now - timeOLEDB;
        double timeOLEDLike = timeSpanOLEDB.TotalMilliseconds;
        System.Console.WriteLine("Time spent via OLEDB(milliseconds) using 'Like' to compare - {0}", timeOLEDLike.ToString());

        System.Console.WriteLine("ODBC is faster than OLEDB {0} times using '=' to compare", Math.Round(timeOLEDBEqual / timeOdbcEqual, 0));
        System.Console.WriteLine("ODBC is faster than OLEDB {0} times using 'Like' to compare", Math.Round(timeOLEDBEqual / timeOdbcEqual, 0));

首次运行后的控制台：

Time spent via ODBC(milliseconds) using '=' to compare - 5,0006
Time spent via ODBC(milliseconds) using 'Like' to compare - 3,5005
Time spent via OLEDB(milliseconds) using '=' to compare - 1630,207
Time spent via OLEDB(milliseconds) using 'Like' to compare - 1755,2228
ODBC is faster than OLEDB 326 times using '=' to compare
ODBC is faster than OLEDB 326 times using 'Like' to compare

Console, after the second run:
Time spent via ODBC(milliseconds) using '=' to compare - 4,5006
Time spent via ODBC(milliseconds) using 'Like' to compare - 4,5005
Time spent via OLEDB(milliseconds) using '=' to compare - 1526,1938
Time spent via OLEDB(milliseconds) using 'Like' to compare - 1595,2026
ODBC is faster than OLEDB 339 times using '=' to compare
ODBC is faster than OLEDB 339 times using 'Like' to compare

Console, after the third run:
Time spent via ODBC(milliseconds) using '=' to compare - 4,0005
Time spent via ODBC(milliseconds) using 'Like' to compare - 3,0004
Time spent via OLEDB(milliseconds) using '=' to compare - 1449,184
Time spent via OLEDB(milliseconds) using 'Like' to compare - 1451,1843
ODBC is faster than OLEDB 362 times using '=' to compare
ODBC is faster than OLEDB 362 times using 'Like' to compare

Console, after the fourth run:
Time spent via ODBC(milliseconds) using '=' to compare - 3,5004
Time spent via ODBC(milliseconds) using 'Like' to compare - 4,5006
Time spent via OLEDB(milliseconds) using '=' to compare - 1475,6874
Time spent via OLEDB(milliseconds) using 'Like' to compare - 1621,2059
ODBC is faster than OLEDB 422 times using '=' to compare
ODBC is faster than OLEDB 422 times using 'Like' to compare

在此示例中，索引字段PRICE_N包含在查询的Where子句中。我还测试了相同的查询，包括Where子句中的非索引字段而不是索引字段。结果是相同的~1400-1600毫秒。我的印象是，在OLEDB（VFPOLEDB）的情况下，不使用索引。我对结果不满意，我需要使用索引。

如果有人有任何建议，我将非常感激。

Answer 1

只是想知道......你的“Price_n”列是NUMERIC还是STRING列。如果是数字，那么我想知道VFP OleDb是否试图将所有数字转换为测试的STRING等价物，而不是将引用的字符串转换为Price_n数据类型的数字等价物，因为我希望自然索引基于

如果是这样，请尝试测试并将所有WHERE子句分别更改为

WHERE price_n = 641857

WHERE price_n赞641857

但是如果基于数字的列，那么Like真的不适用，因为数字是一个数字，而不像像

那样的部分字符串匹配

"1" LIKE "1"
"1" LIKE "10"
"1" LIKE "19302"... etc where they all start with a same value

Answer 2

@Sergiy，你正在做的事情有很大的不同。对于VFP6之后的版本（即2.5或2.6是包含ODBC驱动程序的最后一个包），ODBC驱动程序不存在。 IOW ODBC仅支持VFP6引擎。 OTOH VFPOLEDB支持VFP9引擎（以及所有那些添加了SQL功能的铃声和口哨声）。

在这些引擎之间，存在一个问题，使得查询在文本字段上变慢：如果OS代码页与表的代码页不同，则对表达式为字符类型的索引进行搜索。然后它不使用索引但是进行表扫描。这个＆＃34; bug＆＃34;在VFP9初始释放后浮出水面并且没有更正AFAIK。

根据= vs like，类似于隐式ANSI，所以表现得像＆lt; g>使用==运算符（完全匹配）。使用=，如果ANSI为OFF，则部分匹配为真。

PS：VFPOLEDB即使在修改了代码页之后，也会稍微慢一点但可以忽略不计。

以下是我的代码时序，在一个包含1,000,000行的测试表中：

Time spent via ODBC(milliseconds) using '=' to compare - 41.0023
Time spent via ODBC(milliseconds) using 'Like' to compare - 0
Time spent via OLEDB(milliseconds) using '=' to compare - 68.0038
Time spent via OLEDB(milliseconds) using 'Like' to compare - 2.0002
ODBC is faster than OLEDB 2 times using '=' to compare
ODBC is faster than OLEDB 2 times using 'Like' to compare

这是第二次运行后的时间：

Time spent via ODBC(milliseconds) using '=' to compare - 1
Time spent via ODBC(milliseconds) using 'Like' to compare - 1.0001
Time spent via OLEDB(milliseconds) using '=' to compare - 3.0001
Time spent via OLEDB(milliseconds) using 'Like' to compare - 0
ODBC is faster than OLEDB 3 times using '=' to compare
ODBC is faster than OLEDB 3 times using 'Like' to compare

Answer 3

@Cetin Basoz，

在这些引擎之间，存在使查询变慢的问题文本字段：如果操作系统代码页与表的代码页不同正在对表达式具有特征的索引进行搜索类型。然后它不使用索引，而是进行表扫描。这个“虫子” 在VFP9初始释放后浮出水面并且没有更正AFAIK。

你遇到了问题的关键！

我不知道那种特殊性。我决定检查一下是否真的如此。如果你的假设是对的，低速的原因是DBF文件的代码页和我的操作系统是不同的。为了测试我安装了Visual Fox Pro 9（我以前从未处理过它）并将所有数据传输到新表。然后我在Table Designer中打开了表，并在PRICE_N字段上创建了一个常规索引。因此，新表和我的操作系统的代码页变得相同。

然后我再次运行测试。结果发生了巨大变化。

第一次运行后：

Time spent via ODBC(milliseconds) using '=' to compare - 12,5016
Time spent via ODBC(milliseconds) using 'Like' to compare - 3,5005
Time spent via OLEDB(milliseconds) using '=' to compare - 20,0025
Time spent via OLEDB(milliseconds) using 'Like' to compare - 3,0004
ODBC is faster than OLEDB 2 times using '=' to compare
ODBC is faster than OLEDB 2 times using 'Like' to compare

第二次运行后：

Time spent via ODBC(milliseconds) using '=' to compare - 3,0004
Time spent via ODBC(milliseconds) using 'Like' to compare - 2,5003
Time spent via OLEDB(milliseconds) using '=' to compare - 11,0014
Time spent via OLEDB(milliseconds) using 'Like' to compare - 3,5005
ODBC is faster than OLEDB 4 times using '=' to compare
ODBC is faster than OLEDB 4 times using 'Like' to compare

谢谢，Cetin Basoz。评论很棒：）

即使我不允许更改生产DBF文件的代码页，但至少现在当我知道发生了什么时，我可以放松一下。

Answer 4

@Sergiy，我可能有一个解决方案：

string sqlEq = "SELECT utk_ved FROM prod WHERE Price_N = '641857'";
string sqlLike = "SELECT utk_ved FROM prod WHERE Price_N like '641857'";

TimeSpan timeSpanODBC;
DateTime timeODBC = DateTime.Now;

OdbcConnection odbcConnection = new OdbcConnection(@"Driver={Microsoft Visual FoxPro Driver};SourceType=DBF;SourceDB=C:\Users\Vakshul\Documents\dbfs;Exclusive=No;Collate=Machine;NULL=NO;DELETED=NO;BACKGROUNDFETCH=NO;");
odbcConnection.Open();
OdbcCommand odbcCommand = new OdbcCommand(sqlEq, odbcConnection);
odbcCommand.ExecuteScalar();
timeSpanODBC = DateTime.Now - timeODBC;
double timeOdbcEqual = timeSpanODBC.TotalMilliseconds;
System.Console.WriteLine("Time spent via ODBC(milliseconds) using '=' to compare - {0}", timeOdbcEqual.ToString());


timeODBC = DateTime.Now;

odbcCommand = new OdbcCommand(sqlLike, odbcConnection);
odbcCommand.ExecuteScalar();
timeSpanODBC = DateTime.Now - timeODBC;
double timeOdbcLike = timeSpanODBC.TotalMilliseconds;
System.Console.WriteLine("Time spent via ODBC(milliseconds) using 'Like' to compare - {0}", timeOdbcLike.ToString());

TimeSpan timeSpanOLEDB;
DateTime timeOLEDB = DateTime.Now;

OleDbConnection oleDbCon = new OleDbConnection(@"Provider=VFPOLEDB.1;Data Source=C:\Users\Vakshul\Documents\dbfs;Collating Sequence=MACHINE;Mode=Read");
oleDbCon.Open();
new OleDbCommand("set enginebehavior 80", oleDbCon).ExecuteNonQuery();
OleDbCommand oleDbcommand = new OleDbCommand(sqlEq, oleDbCon);
oleDbcommand.ExecuteScalar();
timeSpanOLEDB = DateTime.Now - timeOLEDB;
double timeOLEDBEqual = timeSpanOLEDB.TotalMilliseconds;
System.Console.WriteLine("Time spent via OLEDB(milliseconds) using '=' to compare - {0}", timeOLEDBEqual.ToString());

timeOLEDB = DateTime.Now;

oleDbcommand = new OleDbCommand(sqlLike, oleDbCon);

oleDbcommand.ExecuteScalar();
timeSpanOLEDB = DateTime.Now - timeOLEDB;
double timeOLEDLike = timeSpanOLEDB.TotalMilliseconds;
System.Console.WriteLine("Time spent via OLEDB(milliseconds) using 'Like' to compare - {0}", timeOLEDLike.ToString());

请注意此行 在同一连接上 ：

new OleDbCommand("set enginebehavior 80", oleDbCon).ExecuteNonQuery();

这不会影响您的结果，但会得到您想要的结果。在我的头顶，它可能影响的唯一地方是“分组依据”查询。认为你不会以旧的越野车VFP方式通过查询来编写小组，你应该是安全的。

我的时间没有“Set EngineBehavior 80”：

Time spent via ODBC(milliseconds) using '=' to compare - 4.0002
Time spent via ODBC(milliseconds) using 'Like' to compare - 1.0001
Time spent via OLEDB(milliseconds) using '=' to compare - 352.0201
Time spent via OLEDB(milliseconds) using 'Like' to compare - 659.0377

并使用“Set EngineBehavior 80”：

Time spent via ODBC(milliseconds) using '=' to compare - 3.0001
Time spent via ODBC(milliseconds) using 'Like' to compare - 2.0002
Time spent via OLEDB(milliseconds) using '=' to compare - 15.0008
Time spent via OLEDB(milliseconds) using 'Like' to compare - 3.0002

使用OLEDB（VFPOLEDB）在DBF文件上运行查询太慢

4 个答案: