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0b0885325c
In sqlite3 generating the loop to read multiple joined tables follows
the pattern:
```c
sqlite3WhereBegin();
sqlite3WhereEnd();
```
and this generates:
```
sqlite> explain select * from users, products;
addr opcode p1 p2 p3 p4 p5 comment
---- ------------- ---- ---- ---- ------------- -- -------------
0 Init 0 23 0 0 Start at 23
1 OpenRead 0 2 0 10 0 root=2 iDb=0; users
2 OpenRead 1 3 0 3 0 root=3 iDb=0; products
3 Rewind 0 22 0 0
4 Rewind 1 22 0 0
5 Rowid 0 1 0 0 r[1]=users.rowid
6 Column 0 1 2 0 r[2]= cursor 0 column 1
7 Column 0 2 3 0 r[3]= cursor 0 column 2
8 Column 0 3 4 0 r[4]= cursor 0 column 3
9 Column 0 4 5 0 r[5]= cursor 0 column 4
10 Column 0 5 6 0 r[6]= cursor 0 column 5
11 Column 0 6 7 0 r[7]= cursor 0 column 6
12 Column 0 7 8 0 r[8]= cursor 0 column 7
13 Column 0 8 9 0 r[9]= cursor 0 column 8
14 Column 0 9 10 0 r[10]= cursor 0 column 9
15 Rowid 1 11 0 0 r[11]=products.rowid
16 Column 1 1 12 0 r[12]= cursor 1 column 1
17 Column 1 2 13 0 r[13]= cursor 1 column 2
18 RealAffinity 13 0 0 0
19 ResultRow 1 13 0 0 output=r[1..13]
20 Next 1 5 0 1
21 Next 0 4 0 1
22 Halt 0 0 0 0
23 Transaction 0 0 2 0 1 usesStmtJournal=0
24 Goto 0 1 0 0
```
`sqlite3WhereBegin()` as the name represents, mainly does stuff with
`WHERE` expressions + loop generation. This is why I decided to change
the name to `translate_tables_begin` to try improve the naming.
In our case:
```rust
translate_table_open_cursor(program, &mut context, select.from.as_ref().unwrap());
translate_table_open_loop(program, &mut context, loop_index);
```
translates into:
```sql
> explain select * from users, products;
addr opcode p1 p2 p3 p4 p5 comment
---- ------------- ---- ---- ---- ------------- -- -------
0 Init 0 28 0 0 Start at 28
1 OpenReadAsync 0 2 0 0 root=2
2 OpenReadAwait 0 0 0 0
3 OpenReadAsync 1 3 0 0 root=3
4 OpenReadAwait 0 0 0 0
5 RewindAsync 0 0 0 0
6 RewindAwait 0 27 0 0
7 RewindAsync 1 0 0 0
8 RewindAwait 1 25 0 0
9 RowId 0 0 0 0
10 Column 0 1 1 0 r[1]= cursor 0 column 1
11 Column 0 2 2 0 r[2]= cursor 0 column 2
12 Column 0 3 3 0 r[3]= cursor 0 column 3
13 Column 0 4 4 0 r[4]= cursor 0 column 4
14 Column 0 5 5 0 r[5]= cursor 0 column 5
15 Column 0 6 6 0 r[6]= cursor 0 column 6
16 Column 0 7 7 0 r[7]= cursor 0 column 7
17 Column 0 8 8 0 r[8]= cursor 0 column 8
18 Column 0 9 9 0 r[9]= cursor 0 column 9
19 RowId 1 10 0 0
20 Column 1 1 11 0 r[11]= cursor 1 column 1
21 Column 1 2 12 0 r[12]= cursor 1 column 2
22 ResultRow 0 13 0 0 output=r[0..13]
23 NextAsync 1 0 0 0
24 NextAwait 1 8 0 0
25 NextAsync 0 0 0 0
26 NextAwait 0 6 0 0
27 Halt 0 0 0 0
28 Transaction 0 0 0 0
29 Goto 0 1 0 0
```
This works on as many joined tables but... it is ready to extend for
further join operations.
Signed-off-by: Pere Diaz Bou <pere-altea@hotmail.com>