Support autoindexing

[jussisaurio]

Sqlite3 uses an automatic ephemeral index to avoid a naive nested loop join when there are no usable indexes.

sqlite> explain select p.name, u.first_name from users u join products p on p.name = u.first_name;
addr  opcode         p1    p2    p3    p4             p5  comment      
----  -------------  ----  ----  ----  -------------  --  -------------
0     Init           0     22    0                    0   Start at 22
1     OpenRead       0     2     0     2              0   root=2 iDb=0; users
2     OpenRead       1     3     0     3              0   root=3 iDb=0; products
3     Rewind         0     21    0                    0   
4       Once           0     12    0                    0   
5       OpenAutoindex  2     2     0     k(2,B,)        0   nColumn=2; for products
6       Rewind         1     12    0                    0   
7         Column         1     1     2                    0   r[2]= cursor 1 column 1
8         Rowid          1     3     0                    0   r[3]=products.rowid
9         MakeRecord     2     2     1                    0   r[1]=mkrec(r[2..3])
10        IdxInsert      2     1     0                    16  key=r[1]
11      Next           1     7     0                    3   
12      Column         0     1     4                    0   r[4]= cursor 0 column 1
13      IsNull         4     20    0                    0   if r[4]==NULL goto 20
14      SeekGE         2     20    4     1              0   key=r[4]
15        IdxGT          2     20    4     1              0   key=r[4]
16        Column         2     0     5                    0   r[5]= cursor 2 column 0
17        Column         0     1     6                    0   r[6]= cursor 0 column 1
18        ResultRow      5     2     0                    0   output=r[5..6]
19      Next           2     15    0                    0   
20    Next           0     4     0                    1   
21    Halt           0     0     0                    0   
22    Transaction    0     0     3     0              1   usesStmtJournal=0
23    Goto           0     1     0                    0