Access Continuous Form with Linked Table - How to Avoid Hitting Database Server for Every Row in Form?

2
votes

I'm migrating the data from an Access database to SQL Server via the SQL Server Migration Assistant (SSMA). The Access application will continue to be used with the local tables converted to linked tables.

One continuous form hangs for 15 - 30 seconds when it's loading. It displays approximately 2000 records. When I looked in SQL Server Profiler to see what it was doing, it was making a separate call to the backend database for each record in the form. So the delay when the form opens is caused by the 2000-odd separate calls to the database.

This is amazingly inefficient. Is there any way to get Access to make a single call to the backend database and retrieve all the records at once?

I don't know if this is relevant but the Record Source for the form is a view in the SQL Server backend database, which is linked to via an Access linked table (so, hopefully, Access just sees it as a table, not a view). I needed an Instead Of trigger on the view in SQL Server, and a unique index on the linked table in Access, to allow the records to be updated via the form.

4
ms-accessms-access-2007

4 Answers

2
votes

If the act of opening that continuous form really does generate ~2000 separate SQL queries (one for every row in the view) then that is unusual behaviour for Access interacting with a SQL Server linked "table". Under normal circumstances what takes place is:

  • Access submits a single query to return all of the Primary Key values for all rows in the table/view. This query may be filtered and/or sorted by other columns based on the Filter and Order By properties of the form. This gives Access a list of the key values for every row that might be displayed in the form, in the order in which they will appear.

  • Access then creates a SQL prepared statement using sp_prepexec to retrieve entire rows from the table/view ten (10) rows at a time. The first call looks something like this...

    declare @p1 int
    set @p1=4
    exec sp_prepexec @p1 output,N'@P1 int,@P2 int,@P3 int,@P4 int,@P5 int,@P6 int,@P7 int,@P8 int,@P9 int,@P10 int',N'SELECT "ID","AgentName" FROM "dbo"."myTbl" WHERE "ID" = @P1 OR "ID" = @P2 OR "ID" = @P3 OR "ID" = @P4 OR "ID" = @P5 OR "ID" = @P6 OR "ID" = @P7 OR "ID" = @P8 OR "ID" = @P9 OR "ID" = @P10',358,359,360,361,362,363,364,365,366,367
    select @p1

    ...and each subsequent call uses sp_execute, something like this

    exec sp_execute 4,368,369,370,371,372,373,374,375,376,377

  • Access repeats those calls until it has retrieved enough rows to fill the current page of continuous forms. It then displays those forms immediately.

  • Once the forms have been displayed, Access will "pre-fetch" a couple of more batches of rows (10 rows each) in anticipation of the user hitting PgDn or starting to scroll down.

  • If the user clicks the "Last Record" button in the record navigator, Access again uses sp_prepexec and sp_execute to request enough 10-row batches to fill the last page of the form, and possibly pre-fetch another couple of batches in case the user decides to hit PgUp or start scrolling up.

So in your case if Access really is causing SQL Server to run individual queries for every single row in the view then there may be something particular about your SQL View that is causing it. You could test that by creating an Access linked table to a single SQL Table or a simple one-table SQL View, then use SQL Server Profiler to check if opening that linked table causes the same behaviour.

2
votes

Turned out the problem was two aggregate fields. One field's Control Source was =Count(ID) and the other field's Control Source was =Sum(Total_Qty).

Clearing the control sources of those two fields allowed the form to open quickly. SQL Server Profiler shows it calling sp_execute, as Gord Thompson described, to retrieve seven batches of 10 rows at a time. Much quicker than making 2000 calls to retrieve one row at a time.

1
votes

I've come across the same problem again but this time with a different cause. I'm including it here for completeness, to help anyone in a similar situation:

This time the underlying query was hanging and SQL Server Profiler showed the same behaviour as before, with Access making separate calls to the SQL Server database to bring back one record at a time, for every record in the query.

The cause turned out to be the ORDER BY clause in the query. I guess Access had to pull back all records in the linked table from SQL Server before being able to order them. Makes sense when I think of it. Although I don't know why Access doesn't just pull all records through at once, instead of getting the records one at a time.

0
votes

I would try setting the Recordset Type to Snapshot (on the Data tab of the Form's property sheet and/or the property sheet of the query you are using for the form source)