C# ADO: Optimize query and its performance

问题

I have a C# RestApi which need to extract information of records from database using ADO by executing queries like this:

declare @id1 int 
set @id1 =  (select id from myTable where col1= x1, col2 = y1, col3 = z1 from mappingTable)

declare @id2 int 
set @id2 =  (select id from myTable where col1= x2, col2 = y2, col3 = z2 from mappingTable)

declare @id3 int 
set @id3 =  (select id from myTable where col1= x3, col2 = y3, col3 = z3 from mappingTable)
.
.
.
declare @idN int 
set @idN =  (select id from myTable where col1= xN, col2 = yN, col3 = zN from mappingTable)

select @id1,@id2,@id3, ..., @idN

I run above query which runs N queries inside it using ADO.NET SqlCommand and read the results. I have two questions:

Does running each of queries using separate SqlCommand lead to performance downgrade or not? Usually in I/O tasks, executing many small I/O tasks have lower performance than running all of them in one batch I/O tasks but I have no idea about the same scenario in Databases and ADO.
Are there any better ways to extract the same result with a better SQL query? In other words can I write this query other way to run it with better performance?

Note: In above queries columns and tables are the same in all queries only values in where clause are modified.

回答1:

Use a table valued parameter to store corresponding values of x, y and z. Use a single query where you inner join your mappingTable with that table valued parameter.

In SQL:

CREATE TYPE XYZ As Table -- Do not call it XYZ!!!
(
    x int,
    y int,
    z int -- I'm using ints because it's simple - you need the data types of your columns here
)

In c#, create a data table that corresponds to this type:

var dt = new DataTable();
dt.Columns.Add("x", typeof(int));
dt.Columns.Add("y", typeof(int));
dt.Columns.Add("z", typeof(int));

populate that data table with the correct values, and then you send the data table to SQL Server as a parameter of type SqlDbType.Structured:

 cmd.Parameters.Add("@tvp", SqlDbType.Structured).Value = dt;

To use it in a query:

SELECT id 
FROM mappingTable
JOIN @tvp
    ON col1= x
   AND col2 = y
   AND col3 = z

This will return a recordset containing a single column with all the ids you need.

Note: Code was written directly here - there might be some typos.

来源：https://stackoverflow.com/questions/59298365/c-sharp-ado-optimize-query-and-its-performance

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