sql - Performance improvement for linq query with distinct -

considering sample table

col 1, col2, col3 1    , x   , g 1    , y   , h 2    , z   , j 2    ,   , k 2    ,   , k 3    , b   , e 

i want below result, i.e distinct rows

1    , x   , g 1    , y   , h 2    , z   , j 2    ,   , k 3    , b   , e 

i tried

var result = context.table.select(c =>                   new {                         col1 = c.col1,                        col2 = c.col2,                        col3 = c.col3                        }).distinct(); 

and

context.table.groupby(x=>new {x.col1,x.col2,x.col3}).select(x=>x.first()).tolist(); 

the results expected, table has 35 columns , 1 million records , size keep on growing, current time query 22-30 secs, how improve performance , down 2-3 secs?

using distinct way go... i'd first approach tried correct 1 - need 1 million rows? see where conditions can add or maybe take first x records?

var result = context.table.select(c => new      {          col1 = c.col1,         col2 = c.col2,         col3 = c.col3      })     .where(c => /*some condition narrow results*/)     .take(1000) //some number of wanted amount of records     .distinct(); 

what might able do, use rownum select in bulks. like:

public <return type> retrievebulk(int fromrow, int torow) {     return context.table.where(record => record.rownum >= fromrow && record.rownum < torow)         .select(c => new          {              col1 = c.col1,             col2 = c.col2,             col3 = c.col3          }).distinct(); } 

this code can like:

list<task<return type>> selecttasks = new list<task<return type>>(); for(int = 0; < 1000000; i+=1000) {     selecttasks.add(task.run(() => retrievebulk(i, + 1000))); }  task.waitall(selecttasks);  //and intercet data using efficient structure hashset when intersect wont o(n)2 o(n)