C# OutOfMemory exception when returning a small Da

I have a SQLite database that has a single table with 18 million rows and 24 columns. I have written a SQL query function in C#, which I expose to Excel with ExcelDNA.

This is the complete code,

    string constr = constr = "Data Source=" + FilePath + ";Version=3;Synchronous=OFF;temp_store=memory;cache_size=700000;count_changes=off;";
    DataTable dt = new DataTable();
    try
    {                                
        SQLiteConnection conn = new SQLiteConnection(constr);
        SQLiteCommand command = new SQLiteCommand(SQLStatement,conn);                
        conn.Open();                
        SQLiteDataAdapter sda = new SQLiteDataAdapter(command);                                
        sda.Fill(dt);
        sda.Dispose();
        command.Dispose();
        conn.Dispose();
        int numRows = (IncludeHeaders ? dt.Rows.Count + 1 : dt.Rows.Count);
        object[,] ret = new object[numRows, dt.Columns.Count];
        int rowCount = 0;
        if (IncludeHeaders)
        {
            int colCount = 0;
            foreach (DataColumn col in dt.Columns)
            {
                ret[rowCount, colCount] = col.ColumnName;
                colCount++;
            }
        }
        rowCount = (IncludeHeaders ? 1 : 0);
        foreach (DataRow row in dt.Rows)
        {
            int colCount = 0;
            foreach (DataColumn col in dt.Columns)
            {
                if (row[col] != DBNull.Value)
                    ret[rowCount, colCount] = row[col];
                else
                    ret[rowCount, colCount] = "";
                colCount++;
            }
            rowCount++;
        }
        return ret;
    }
    catch (Exception ex)
    {
        object[,] err = new object[1, 1];
        err[0, 0] = ex.ToString();
        return err;
    }
    finally
    {
        dt.Clear();
        dt.Dispose();
        dt = null;  
    }

If I run the query twice (two hits of Shift+F9), I get an OutOfMemoryException. In the Task Manager I can see the Working Set(Memory) of the EXCEL.EXE image go from 200MB to 1500MB before the exception is thrown.

However, this behavior isn't entirely consistent. Other queries where I return upto 5 columns and 1100 rows work just fine. I see the memory usage tick up in the Task Manager and once the results are returned to Excel I see the memory come back down.

Debugging the application above shows that it trips up at the sda.Fill(dt) line.

Would appreciate any thoughts? Would I be better off using SQLiteDataReader instead? Or are there any other tips or tricks I can use? Thank you.

Incidentally if I run the exact query via Python I don't get this problem, so I'd assume it's something to do with the garbage collection in C#.

Here are some details on the database and query. The schema is along the lines of,

Date (VARCHAR)
CompanyName (VARCHAR)
Amount (REAL)
AggCode (VARCHAR)
Level1 ... Level20 (VARCHAR)

The queries are usually run combining the fields Level9, Level5, AggCode, Date, CompanyName in the WHERE clause. So apart from the raw table, I have also configured the following four indices,

CREATE INDEX idx1 on my(Level09, AggCode);
CREATE INDEX idx2 on my(Level05, AggCode);
CREATE INDEX idx3 on my(CompanyName, AggCode);
CREATE INDEX idx4 on my(Date, AggCode);

The query that returns 1100 rows and 2 columns successfully,

SELECT CompanyName, SUM(Amount) FROM my where Level09="T_EU" and AggCode = "R_S_V" GROUP BY CompanyName ORDER BY SUM(Amount) DESC

The query that throws the memory exception,

SELECT Date, CompanyName, Sum(Amount) FROM my WHERE Level05 ="M_TO" AND AggCode = "C_DTA" GROUP BY Date, CompanyName

The second query returns 163 rows and 3 columns in Python.

The full stack trace of the exception is below,

System.Data.SQLite.SQLiteException (0x80004005): out of memory
out of memory
at System.Data.SQLite.SQLite3.Reset(SQLiteStatement stmt)
at System.Data.SQLite.SQLite3.Step(SQLiteStatement stmt)
at System.Data.SQLite.SQLiteDataReader.NextResult()
at System.Data.SQLite.SQLiteDataReader..ctor(SQLiteCommand cmd, CommandBehavior behave)
at System.Data.SQLite.SQLiteCommand.ExecuteReader(CommandBehavior behavior)
at System.Data.SQLite.SQLiteCommand.ExecuteDbDataReader(CommandBehavior behavior)
at System.Data.Common.DbCommand.System.Data.IDbCommand.ExecuteReader(CommandBehavior behavior)
at System.Data.Common.DbDataAdapter.FillInternal(DataSet dataset, DataTable[] datatables, Int32 startRecord, Int32 maxRecords, String srcTable, IDbCommand command, CommandBehavior behavior)
at System.Data.Common.DbDataAdapter.Fill(DataTable[] dataTables, Int32 startRecord, Int32 maxRecords, IDbCommand command, CommandBehavior behavior)
at System.Data.Common.DbDataAdapter.Fill(DataTable dataTable)
at UtilXL.Utils.UtilsSQLite.RunQueryCSLite(String SQLStatement, String FilePath, Boolean IncludeHeaders) in h:\Projects\UtilXL\UtilXL\Utils\UtilsSQLite.cs:line 37

Line 37 in referenced above is the sda.Fill() call.

If I use SqlDataReader then it falls over at the ExecuteReader() command,

System.Data.SQLite.SQLiteException (0x80004005): out of memory
at System.Data.SQLite.SQLite3.Reset(SQLiteStatement stmt)
at System.Data.SQLite.SQLite3.Step(SQLiteStatement stmt)
at System.Data.SQLite.SQLiteDataReader.NextResult() 
at System.Data.SQLite.SQLiteDataReader..ctor(SQLiteCommand cmd, CommandBehavior behave)
at System.Data.SQLite.SQLiteCommand.ExecuteReader(CommandBehavior behavior)
at System.Data.SQLite.SQLiteCommand.ExecuteReader()
at UtilXL.Utils.UtilsSQLite.RunQueryCSReader(String SQLStatement, String FilePath, Boolean IncludeHeaders) in h:\Projects\UtilXL\UtilXL\Utils\UtilsSQLite.cs:line 111

Your code does not have anything which can cause OutOfMemoryException. So possibility is huge datatable and memory not getting recovered immediately. One possibility is - As you are reading excel.. it may have blank rows at the end, which may potentially can increase dt size.....

As datatable is managed object calling dispose will not really help.. But at lease you can clear dt everytime once it use is over..