sqlite> EXPLAIN select count(distinct t1) from temp2; 
addr  opcode         p1    p2    p3    p4             p5  comment        
----  -------------  ----  ----  ----  -------------  --  -------------  
0     Init           0     15    0                    0                  
1     Null           0     1     2                    0                  
2     OpenEphemeral  1     0     0     k(1,B)         0                  
3     OpenRead       0     2     0     1              0                  
4     Rewind         0     11    0                    0                  
5       Column         0     0     3                    0
6       Found          1     10    3     1              0
7       MakeRecord     3     1     4                    0
8       IdxInsert      1     4     3     1              16
9       AggStep        0     3     1     count(1)       1
10    Next           0     5     0                    1                  
11    AggFinal       1     1     0     count(1)       0                  
12    Copy           1     5     0                    0                  
13    ResultRow      5     1     0                    0                  
14    Halt           0     0     0                    0                  
15    Transaction    0     0     1     0              1                  
16    Goto           0     1     0                    0                  

limbo> EXPLAIN select count(distinct t1) from temp2;
addr  opcode             p1    p2    p3    p4             p5  comment
----  -----------------  ----  ----  ----  -------------  --  -------
0     Init               0     13    0                    0   Start at 13
1     OpenReadAsync      0     2     0                    0   table=temp2, root=2
2     OpenReadAwait      0     0     0                    0
3     RewindAsync        0     0     0                    0
4     RewindAwait        0     9     0                    0   Rewind table temp2
5       Column           0     0     3                    0   r[3]=temp2.t1
6       AggStep          0     3     2     count          0   accum=r[2] step(r[3])
7     NextAsync          0     0     0                    0
8     NextAwait          0     5     0                    0
9     AggFinal           0     2     0     count          0   accum=r[2]
10    Copy               2     1     0                    0   r[1]=r[2]
11    ResultRow          1     1     0                    0   output=r[1]
12    Halt               0     0     0                    0
13    Transaction        0     0     0                    0
14    Goto               0     1     0                    0