Technical Information Database
TI850D.txt Extracting Index Data From A Table
Category :Database Programming
Platform :All
Product :Delphi All
Description:
Getting a list of the indexes associated with a table at run-time can be
as simple as a call to the GetIndexNames method of the TTable, TQuery, or
TStoredProc component. The GetIndexNames method returns a list of the
that are available for the data set in the form of a TStrings list, which
may then be inserted into such visual components as a TListBox through its
Items property:
ListBox1.Clear;
Table1.GetIndexNames(ListBox1.Items);
Of course, the TStrings list returned by the GetIndexNames method need not
be used with a visual component. It could just as well serve as an array
of index names stored entirely in memory, that can be used as a list or
array.
But it is also possible to retrieve much more information about the
indexes for a table than just the names. Other descriptive attributes
include the name of each index, the names of the fields that comprise each
index, and the index options used when each index was created. Retrieving
these values is slightly more involved than the use of the GetIndexNames.
Basically, this process involves iterating through the IndexDefs property
of the TTable, TQuery, or TStoredProc component. The IndexDefs property is
essentially an array of records, one record for each index for the table.
Each index record contains information about the index. It is a relatively
straightforward process to iterate through this array of index descrip-
tions, extracting information about individual indexes.
The IndexDefs property of the TTable component contains information about
the indexes for the table used by the TTable, TQuery, or TStoredProc comp-
onent in use. The IndexDefs property itself has various properties that
allow for the extraction of information about specific indexes. The two
properties in the IndexDefs object are:
Count: type Integer; available only at run-time and read-only; indicates
the number ofentries in the Items property (i.e., the number of
indexes in the table).
Items: type TIndexDef; available only at run-time and read-only; an
array of TIndexDef objects, one for each index in the table.
The Count property of the IndexDefs object is used as the basis for a
loop program construct to iterate through the Items property entries to
extract specific information about each index. Each IndexDef object con-
tained in the Items property consists of a number of properties that pro-
vide various bits of information that describe each index. All of the
properties of the IndexDef object are available only at run-time and are
all read-only. These properties are:
Expression: type String; indicates the expression used for dBASE multi-
field indexes.
Fields: type String; indicates the field or fields upon which the
index is based.
Name: type String; name of the index.
Options: type TIndexOptions; characteristics of the index (ixPrimary,
ixUnique, etc.).
Before any index information (Count or Items) can be accessed, the Update
method of the IndexDefs object must be called. This refreshes or init-
ializes the IndexDef object's view of the set of indexes.
Examples
========
Here is a simple For loop based on the Count property of the IndexDefs
object that extracts the name of each index (if any exist) for the table
represented by the TTable component Table1:
procedure TForm1.ListBtnClick(Sender: TObject);
var
i: Integer;
begin
ListBox1.Items.Clear;
with Table1 do begin
if IndexDefs.Count > 0 then begin
for i := 0 to IndexDefs.Count - 1 do
ListBox1.Items.Add(IndexDefs.Items[i].Name)
end;
end;
end;
Below is an example showing how to extract information about indexes at
run-time, plugging the extracted values into a TStringGrid (named SG1).
procedure TForm1.FormShow(Sender: TObject);
var
i: Integer;
S: String;
begin
with Table1 do begin
Open;
{Refresh IndexDefs object}
IndexDefs.Update;
if IndexDefs.Count > 0 then begin
{Set up columns and rows in grid to match IndexDefs items}
SG1.ColCount := 4;
SG1.RowCount := IndexDefs.Count + 1;
{Set grid column labels to TIndexDef property names}
SG1.Cells[0, 0] := 'Name';
SG1.ColWidths[0] := 200;
SG1.Cells[1, 0] := 'Fields';
SG1.ColWidths[1] := 200;
SG1.Cells[2, 0] := 'Expression';
SG1.ColWidths[2] := 200;
SG1.Cells[3, 0] := 'Options';
SG1.ColWidths[3] := 300;
{Loop through IndexDefs.Items}
for i := 0 to IndexDefs.Count - 1 do begin
{Fill grid cells for current row}
SG1.Cells[0, i + 1] := IndexDefs.Items[i].Name;
SG1.Cells[1, i + 1] := IndexDefs.Items[i].Fields;
SG1.Cells[2, i + 1] := IndexDefs.Items[i].Expression;
if ixPrimary in IndexDefs.Items[i].Options then
S := 'ixPrimary, ';
if ixUnique in IndexDefs.Items[i].Options then
S := S + 'ixUnique, ';
if ixDescending in IndexDefs.Items[i].Options then
S := S + 'ixDescending, ';
if ixCaseInsensitive in IndexDefs.Items[i].Options then
S := S + 'ixCaseInsensitive, ';
if ixExpression in IndexDefs.Items[i].Options then
S := S + 'ixExpression, ';
if S > ' ' then begin
{Get rid of trailing ", "}
System.Delete(S, Length(S) - 1, 2);
SG1.Cells[3, i + 1] := S;
end;
end;
end;
end;
end;
Special Considerations
======================
There are idiosyncracies associated with extracting information about
indexes for different table types that Delphi can access.
dBASE Tables
------------
With dBASE indexes, which properties of Fields and Expression will be
filled will depend on the type of index, simple (single-field) or
complex (based on multiple fields or a dBASE expression). If the index
is a simple one, the Fields property will contain the name of the field
in the table on which the index is based and the Expression property will
be blank. If the index is a complex one, the Expression property will
show the expression on which the index is based (e.g., "Field1+Field2")
and the Fields property will be blank.
Paradox Tables
--------------
With Paradox primary indexes, the Name property will be blank, the Fields
property will contain the field(s) on which the index is based, and the
Options property will contain ixPrimary. With secondary indexes, the Name
property will contain the name of the secondary index, the Fields prop-
erty will contain the field(s) on which the index is based, and the
Options property may or may not have values.
The Fields property for indexes based on more than one field will show
the field names separated by semi-colons. Indexes based on only a single
field will show the name of only that one field in the Fields property.
InterBase Tables
----------------
For both index types, single- or multiple-field, the Expression property
will be blank. For single-field indexes, the Fields property will contain
the field on which the index is based. For multi-field indexes, the Fields
property will show all of the multiple fields that comprise the index,
each separated by a semi-colon.
Indexes designated as PRIMARY when the CREATE TABLE command is issued will
have "RDB$PRIMARYn" in the Name property, where n is a number character
uniquely identifying the primary index within the database metadata.
Secondary indexes will show the actual name of the index.
Foreign key constraints also result in an index being created by the sys-
tem. These indexes appear in the IndexDefs property, and will have the
name "RDB$FOREIGNn" where n is a number character that uniquely identifies
the index within the database metadata.
The Fields property for indexes based on more than one field will show
the field names separated by semi-colons. Indexes based on only a single
field will show the name of only that one field in the Fields property.
Reference:
7/16/98 4:33:55 PM
Last Modified: 01-SEP-99