join on column with different data types

Hi.
I have a query where a single column is used as join key.
In TableA the column is nvarchar and in TableB it is varchar.
TableB has a clustered index on that column but the join will do a full
table scan
on TableB.
When I convert the coresponding column in TableA to varchar the query use
the index.
The join query will not do an implicit convert between varchar and
nvarchar.
Perhaps as designed.
Where can I find documentation on this issue.
I have been searching books online and Googles but haven't found it.
Sqlserver 2000 or 7.0
--
/dg
----
Dan van Ginhoven
SchlumbergerSema AB
S-412 97 GÖTEBORG Sweden
Phone +46 317 51 44 13
Mob/Cell +46 708 51 44 13convert(varchar(10, column_tableA) = column_tableB
"Dan van Ginhoven" <nospam@.got.sema.se> wrote in message
news:uwiH8IuPDHA.2480@.tk2msftngp13.phx.gbl...
> Hi.
> I have a query where a single column is used as join key.
> In TableA the column is nvarchar and in TableB it is varchar.
> TableB has a clustered index on that column but the join will do a full
> table scan
> on TableB.
> When I convert the coresponding column in TableA to varchar the query use
> the index.
> The join query will not do an implicit convert between varchar and
> nvarchar.
> Perhaps as designed.
> Where can I find documentation on this issue.
> I have been searching books online and Googles but haven't found it.
> Sqlserver 2000 or 7.0
> --
> /dg
> ----
> Dan van Ginhoven
> SchlumbergerSema AB
> S-412 97 GÖTEBORG Sweden
> Phone +46 317 51 44 13
> Mob/Cell +46 708 51 44 13
>|||There is a chart which shows which data types are implicitly or explicitly
convertable in books on line search for convert... The chart indicates that
SQL can implicitly convert between nchar/nvarchar and char/varchar
"Dan van Ginhoven" <nospam@.got.sema.se> wrote in message
news:uwiH8IuPDHA.2480@.tk2msftngp13.phx.gbl...
> Hi.
> I have a query where a single column is used as join key.
> In TableA the column is nvarchar and in TableB it is varchar.
> TableB has a clustered index on that column but the join will do a full
> table scan
> on TableB.
> When I convert the coresponding column in TableA to varchar the query use
> the index.
> The join query will not do an implicit convert between varchar and
> nvarchar.
> Perhaps as designed.
> Where can I find documentation on this issue.
> I have been searching books online and Googles but haven't found it.
> Sqlserver 2000 or 7.0
> --
> /dg
> ----
> Dan van Ginhoven
> SchlumbergerSema AB
> S-412 97 GÖTEBORG Sweden
> Phone +46 317 51 44 13
> Mob/Cell +46 708 51 44 13
>|||Hi Wayne!
Yes I have seen the chart. It surprises me a bit that the query didn't use
the index.
It may be bug.
I´m looking for a text that describes in what situations the Query Planner
will not use an
index, but will do a full table scan. I think I have seen it once.
One example is when a query contains <column> like '%value%'
it won´t use an index on that column to solve the query.
/dg|||> Yes I have seen the chart. It surprises me a bit that the query didn't use
> the index.
> It may be bug.
Nope, it is how SQL Server works. Although you don't have to write any code
for an implicit conversion SQL Server still converts one datatype to another
when it creates the execution plan. If you run the following code in Query
Analyzer and look at the execution plan you will see that #B.B is converted
before the two tables are joined:
CREATE TABLE #A (A nvarchar(20))
GO
CREATE TABLE #B (B varchar(20))
GO
SELECT * FROM #A
INNER JOIN #B
ON #A.A = #B.B
GO
DROP TABLE #A, #B
GO
Why is #B.B converted and not #A.A? That is determined by the Data Type
Precedence. varchar has a lower Data Type Precedence than nvarchar, so
varchar gets converted. (You can find the complete list of the data type
precedence in Books Online under Data Type Precedence).
Because #B.B is used in a function (when it is converted), the Query
Optimizer can't use any indexes on the column and has to use a table scan.
As you saw when the other (nvarchar) column in the join is explicitly
converted, the varchar column won't be implicitly converted and the index on
the varchar column can be used.
About a text: Kalen Delaney has written a series of articles for SQL Server
Magazine (www.sqlmag.com) about which search conditions can make use of
indexes and which don't, and there is also a bit about it in her book Inside
SQL Server 2000.
--
Jacco Schalkwijk MCDBA, MCSD, MCSE
Database Administrator
Eurostop Ltd.
"Dan van Ginhoven" <nospam@.got.sema.se> wrote in message
news:uPHxWevPDHA.1720@.TK2MSFTNGP11.phx.gbl...
> Hi Wayne!
> Yes I have seen the chart. It surprises me a bit that the query didn't use
> the index.
> It may be bug.
> I´m looking for a text that describes in what situations the Query
Planner
> will not use an
> index, but will do a full table scan. I think I have seen it once.
> One example is when a query contains <column> like '%value%'
> it won´t use an index on that column to solve the query.
> /dg
>