Do ADO.Net DataTables have indexes?

Question

I am using VSTS 2008 + C# + .Net 3.5 + SQL Server 2008 + ADO.Net. If I load a table from a database by using a DataTable of ADO.Net, and in the database table, I defined a c

Answer 1

George,

The answer is no.

Actually, some sort of indexing may be used internally, but only as an implementation detail. For instance, if you create a foreign key constraint, maybe that's assisted by an index. But it doesn't matter to a developer.

Answer 2

Actually George's question is not so "bad" as some people insist it is. (I am more and more convinced that there's no such think as "a bad question".)

I have a rather big table which I load into the memory, in a DataTable object. A lot of processing is done on lines from this table, a lot of times, on various (and different) subsets which I can easily describe as "WHERE ..." of SELECT clauses. Now with this DataTable I can run Select() - a method of DataTable class - but it is quite inefficient.

In the end, I decided to load the DataTable sorted by specific columns and implemented my own quick search, instead of using the Select() function. It proved to be much faster, but of course it works only on those sorted columns. The trouble would have been avoided, had a DataTable had indexes.

Answer 3

John above is correct. DataTables are disconnected in memory structures. They do not map to the physical implementation of the database.

The indexes on disk are used to speed up lookups because you don't have all the rows. If you have to load every row and scan them it is slow, so an index makes sense. In a DataTable you already have all the rows, so a comparison is fast already.

Answer 4

The correct answer here is to create a DataView from the DataTable, which according to the doc will create an index:

DataView constructs an index. An index contains keys built from one or more columns in the table or view. These keys are stored in a structure that enables the DataView to find the row or rows associated with the key values quickly and efficiently. Operations that use the index, such as filtering and sorting, see signifcant performance increases. The index for a DataView is built both when the DataView is created and when any of the sorting or filtering information is modified. Creating a DataView and then setting the sorting or filtering information later causes the index to be built at least twice: once when the DataView is created, and again when any of the sort or filter properties are modified.

Answer 5

DataTables have a PrimaryKey field that can serve as an index (they are fast already anyway). This field is not copied from the Primary Keys of the database (although that might be nice).

Answer 6

No, but possibly yes.

You can set up your own indices on a DataTable, using a DataView. As you change the table, the DataView will be rebuilt, so the index should always be up to date.

I did some bench tests for my own app. I use a DataTable to approximate a Boost MultiIndexContainer. To create an index on a column call "Author", I initialise the DataTable, and then the DataView...

_dvChangesByAuthor = 
    new DataView(
        _dtChanges, 
        string.Empty, 
        "Author ASC", 
        DataViewRowState.CurrentRows);

To then pull data by Author from the table, you use the view's FindRows function...

            dataRowViews = _dvChangesByAuthor.FindRows(author);
            List<DataRow> returnRows = new List<DataRow>();
            foreach (DataRowView drv in dataRowViews)
            {
                returnRows.Add(drv.Row);
            }

I made a random large DataTable, and ran queries using DataTable.Select(), Linq-To-DataSet (with forced execution by exporting to list) and the above DataView method. The DataView method won easily. Linq took 5000 ticks, Select took over 26000 ticks, DataView took 192 ticks...

LOC=20141121-14:46:32.863,UTC=20141121-14:46:32.863,DELTA=72718,THR=9,DEBUG,LOG=Program,volumeTest() - Running queries for author >TFYN_AUTHOR_047<
LOC=20141121-14:46:32.863,UTC=20141121-14:46:32.863,DELTA=72718,THR=9,DEBUG,LOG=RightsChangeTracker,GetChangesByAuthorUsingLinqToDataset() - Query elapsed time: 2 ms, 4934 ticks; Rows=65 
LOC=20141121-14:46:32.879,UTC=20141121-14:46:32.879,DELTA=72733,THR=9,DEBUG,LOG=RightsChangeTracker,GetChangesByAuthorUsingSelect() - Query elapsed time: 11 ms, 26575 ticks; Rows=65 
LOC=20141121-14:46:32.879,UTC=20141121-14:46:32.879,DELTA=72733,THR=9,DEBUG,LOG=RightsChangeTracker,GetChangesByAuthorUsingDataview() - Query elapsed time: 0 ms, 192 ticks; Rows=65

So, if you want indices on a DataTable, I would suggest DataView, if you can deal with the fact that the index is re-built when the data changes.