Access Cookbook, 2nd Edition [Electronic resources] نسخه متنی

Recipe 8.2 Make Slow Forms Run Faster

8.2.1 Problem

You are not happy with the
speed at which your forms load and display. How can you change your
forms so they will load and display faster?

8.2.2 Solution

Access gives you a lot of flexibility to develop dynamite-looking
forms. Unfortunately, Access also makes it easy to create forms that
run painfully slowly. The Solution in Recipe 8.1 explained how you can speed up the loading
time of all forms by preloading them. This solution discusses how to
track down and fix various performance bottlenecks, thus improving
form execution performance. We also discuss the use and misuse of
graphic elements and combo and list box controls.

You should consider several potential issues when analyzing your
forms for performance. We discuss here two common performance
bottlenecks: controls involving graphic or memo field data, and combo
and list box controls.

8.2.2.1 Graphic and memo controls

Load the

08-02a.MDB database. Open the
frmCategoriesOriginal form (see Figure 8-5). This
form, although attractive, loads slowly and has a noticeable delay on
slower machines when moving from record to record. Now open
frmCategoriesStep3, which is the final version of the form after
various optimizations have been applied to it (see Figure 8-6). Its load and execution times should be
noticeably faster.

Figure 8-5. The original form, frmCategoriesOriginal, is slow

Figure 8-6. The final form, frmCategoriesStep3, is faster

Follow these steps to improve the
performance of forms that include unbound graphic controls or bound
controls that hold OLE or memo fields:

1. Open the problem form in design view. If you have any unbound object
   frame controls (also know as unbound OLE controls) that are used to
   store fixed graphic images, change them to image controls by
   right-clicking on the object and selecting Change To Image (see Figure 8-7). The frmCategoriesStep1 form in the
   

   08-02a.MDB sample database is identical to
   frmCategoriesOriginal except that ctlLogo has been converted from an
   unbound object frame control to an image control.

Figure 8-7. Changing an unbound object frame control to an image control

2. If
   you created a watermark for the form, consider removing it. To do
   this, select the word "bitmap" in
   the form's Picture property, press the Del key, and
   answer Yes to the confirming dialog. The frmCategoriesStep2 form in
   

   08-02a.MDB is identical to frmCategoriesStep1,
   except that we deleted the watermark.

3. If your form contains any bound controls that hold either OLE or memo
   fields, consider moving the controls to a second page of the form. In
   the final version of the Categories form, named frmCategoriesStep3
   (Figure 8-6), we moved the ctlDescription and
   ctlPicture controls to a second page.

8.2.2.2 Combo and list box controls

Load the

08-02b.MDB database. Open the frmSurveySlow
form. This form contains a combo box control, cboPersonId, that has
as its row source a SQL Select statement that
pulls in 15,000 rows from the tblPeople table. Load time for the form
is slow because Access has to run the query that supplies the 15,000
rows to cboPersonId. Tab to the cboPersonId control and type
"th" to search for the name
"Thompson, Adrian" (see Figure 8-8).

Figure 8-8. The cboPersonId combo box in frmSurveySlow is very slow

Note the long delay before the "th"
list of records appears. Now open the frmSurveyFast form (see Figure 8-9); its load time is significantly faster. Press
the ">" command button to open
the frmPersonPopup form. Type "th"
in the first field and press Tab.

Figure 8-9. In frmSurveyFast, the combo box is replaced with a text box and command button

After a short delay, you'll be able to select
"Thompson, Adrian" from the
drop-down list as shown in Figure 8-10. Press the OK
button, which will drop the chosen name back into the txtPersonName
text box on frmSurveyFast.

Figure 8-10. Selecting a name from the drop-down list is much faster

Follow these steps to improve the speed of forms containing combo or
list boxes that must display a lot of information:

1. Make a copy of the problem form and open the copy in design view.
   Select the slow combo or list box control. Right-click on the control
   and select Change To Text Box.

2. Create a new unbound pop-up form
   with the property settings shown in Table 8-2.
   Leave the remaining property settings at their defaults. In the
   sample database, this form is named frmPersonPopup.

3. Create four unbound controls on this form: a text box, a combo box,
   and two command buttons. In the sample database, we created the
   controls shown in Table 8-3. The text box will be
   used to limit the number of items in the combo box, using the
   parameter query created in Step 4.

4. Create a new query that will serve as the row source for the combo
   box of the pop-up form. If you used a query as the source for the
   combo or list box on the original form, you should be able to modify
   its design. Add the necessary fields to the query. Add a parameter to
   the form that limits the rows based on a value typed into the text
   box on the pop-up form. Choose any sort fields. In the sample
   database, we created the qryPersonComboBox query with the fields
   shown in Table 8-4. Save and close the query.

5. Reopen the pop-up form
   created in Steps 2 and 3. Set the Enabled property of the combo box
   to No. Set the RowSource property to point to the query created in
   Step 4. In the sample database, we set the properties of the
   cboPersonId combo box to the values in Table 8-5.

6. Create a new event procedure for
   the text box's Change event. (If
   you're unsure of how to do this, see
   How Do I Create an Event
   Procedure? in the Preface of this book.) Add the
   following code to the event procedure:

   Private Sub txtChar_Change( )
   If Not IsNull(Me.txtChar.Text) Then
   Me.cboPersonID.Enabled = True
   Else
   Me.cboPersonID.Enabled = False
   End If
   End Sub

   Change txtChar to the name of your text box and cboPersonId to the
   name of your combo box.

7. Create a new event procedure
   for the text box's AfterUpdate event and add the
   following code to it:

   Private Sub txtChar_AfterUpdate( )
   Dim ctlPersonId As ComboBox
   Dim ctlChar As TextBox
   Set ctlPersonId = Me.cboPersonID
   Set ctlChar = Me.txtChar
   If Not IsNull(ctlChar) Then
   ctlPersonId.RowSource = "qryPersonComboBox"
   ctlPersonId.SetFocus
   ctlPersonId.Dropdown
   End If
   End Sub

   Change txtChar to the name of your text box, and cboPersonId to the
   name of your combo box. Change qryPersonComboBox to the name of the
   query you created in Step 4.

8. Create the following new event
   procedure for the OK command button's Click event:

   Private Sub cmdOK_Click( )
   Me.Visible = False
   End Sub

9. Create the following new event procedure for the Cancel command
   button's Click event:

   Private Sub cmdCancel_Click( )
   DoCmd.Close acForm, Me.Name
   End Sub

10. Save the pop-up form and close it.

11. Reopen the form from Step 1 in design view. Add a button called
    cmdPopup to the right of the text box. Add the following event
    procedure to cmdPopup's Click event:

    Private Sub cmdPopup_Click( )
    Const acbcPopup = "frmPersonPopup"
    ' Open up pop-up form in dialog mode.
    DoCmd.OpenForm acbcPopup, WindowMode:=acDialog
    ' Check if form is still loaded.
    ' If yes, then OK button was used to close pop-up.
    If SysCmd(acSysCmdGetObjectState, acForm, acbcPopup) <> 0 Then
    Me.PersonID = Forms(acbcPopup)!cboPersonID
    DoCmd.Close acForm, acbcPopup
    End If
    End Sub

    Change frmPersonPopup to match the name of the pop-up form. Change
    PersonId and cboPersonId to the names of the appropriate controls.

8.2.3 Discussion

When you have a form that loads and executes slowly, you need to
analyze the form and weigh the advantages and disadvantages of using
graphic features. After a careful analysis of the
frmCategoriesOriginal form in the

08-02a.MDB
database, we made several changes.

First, we changed the unbound
object frame control to an image control. The OLE-based object frame
control can be used to hold graphic images, sound, and other
OLE-based data such as Excel spreadsheets or Word documents. But if
you need to display only an unbound bitmap, you're
better off using the more resource-conservative image control.

Second, we removed the form watermark, as this feature slows down
form execution slightly. The improvement in performance depends on
the color-depth of the removed image and the speed of your machine.

Finally, we
created a second page and moved the text box bound to the memo field
and the bound object frame bound to the OLE field to this second
page. These field types (memo and OLE) are stored separately from the
rest of the fields in a record and thus require additional disk reads
to display. Fortunately, Access does not fetch these potentially
large fields from the database unless they are visible on the screen.
By placing them on the second page, you can quickly navigate from row
to row without having to fetch the memo or OLE data. When you need to
view the data in the fields, you can easily flip to the second page
of the form.

The frmSurveySlow form in

08-02b.MDB contains a combo box, cboPersonId,
bound to a 15,000-row table. This makes form load and combo box list
navigation slow. Combo and list box controls are excellent for
allowing users to choose from a list of values and work well with a
small number of list rows. However, they perform poorly when the size
of the list exceeds a few thousand rows, even with very fast
hardware.

We were able to improve the load time of the survey form
significantly by limiting the rows in the person combo box. This was
done using a pop-up form containing the same combo box control, but
linked to a text box control that filtered the combo
box's rows via a parameter query. Using a little VBA
code, we disabled the combo box control until at least one character
was entered into the text box. In this way, we reduced a 15,000-row
combo box to, on average, 577 rows (15000 / 26), and
that's when only the minimum number of characters
(one) is typed into the text box. You could increase performance by
waiting for at least two or even three characters, rather than
filling the list after the user has typed only one letter.

Besides reducing the number of rows in the
row source for cboPersonId, two other improvements were made to boost
combo box performance. On the original frmSurveySlow form, a SQL
statement was used as the row source for the combo box; the
cboPersonId combo box on the pop-up form uses a saved query instead.
Saved queries are always faster than SQL statements because the query
optimizer optimizes the query when it is saved instead of when it is
run.

In addition, the SQL statement for
frmSurveySlow's combo box includes the following
ORDER BY clause:

ORDER BY [LastName] & ", " & [FirstName]

In contrast, the SQL statement for the qryPersonComboBox query used
as the row source for frmPersonPopup uses the following
ORDER BY clause:

ORDER BY tblPeople.LastName, tblPeople.FirstName

Although these two ORDER BY
clauses look similar, the first one sorts on an expression, whereas
the second sorts on two indexed fields. It's always
faster to sort on individual fields rather than expressions.

There are several other things to consider when looking for ways to
speed up your forms. You may wish to try some or all of the following
suggestions:

• Preload and keep loaded forms (see the Solution in Recipe 8.1).

• Ensure that fields used to sort or filter rows are indexed in the
  underlying tables (see the Solution in Recipe 8.4 for more on indexing and its effect on query
  performance).

• Use referential integrity throughout your
  database. Besides the obvious improvements to the quality of your
  data when you create enforced relationships, Access creates hidden
  foreign key indexes that improve the performance of queries, forms,
  and reports based on the joined tables.

• Create simpler forms with less color, fewer graphics, and fewer fonts.

• Limit the number of records in the
  form's recordset (see the Solution in Recipe 8.6).

• Watch out for Tab controls with many
  pages and subforms on each page. Loading all those subforms will slow
  the opening of your form. One alternative is to load the subforms on
  a Tab control page only when that page is selected. You can do this
  by using the Change event of the Tab control to check the Value of
  the controlthis tells you the PageIndex of the selected page.
  You can set the SourceControl property of your subforms only when the
  page they appear on is selected; you can't set it in
  design view.