html_url,issue_url,id,node_id,user,created_at,updated_at,author_association,body,reactions,issue,performed_via_github_app
https://github.com/simonw/datasette/issues/1293#issuecomment-899915829,https://api.github.com/repos/simonw/datasette/issues/1293,899915829,IC_kwDOBm6k_c41o6A1,9599,2021-08-17T01:02:35Z,2021-08-17T01:02:35Z,OWNER,"New approach: this time I'm building a simplified executor for the bytecode operations themselves.
```python
def execute_operations(operations, max_iterations = 100, trace=None):
    trace = trace or (lambda *args: None)
    registers: Dict[int, Any] = {}
    cursors: Dict[int, Tuple[str, Dict]] = {}
    instruction_pointer = 0
    iterations = 0
    result_row = None
    while True:
        iterations += 1
        if iterations > max_iterations:
            break
        operation = operations[instruction_pointer]
        trace(instruction_pointer, dict(operation))
        opcode = operation[""opcode""]
        if opcode == ""Init"":
            if operation[""p2""] != 0:
                instruction_pointer = operation[""p2""]
                continue
            else:
                instruction_pointer += 1
                continue
        elif opcode == ""Goto"":
            instruction_pointer = operation[""p2""]
            continue
        elif opcode == ""Halt"":
            break
        elif opcode == ""OpenRead"":
            cursors[operation[""p1""]] = (""database_table"", {
                ""rootpage"": operation[""p2""],
                ""connection"": operation[""p3""],
            })
        elif opcode == ""OpenEphemeral"":
            cursors[operation[""p1""]] = (""ephemeral"", {
                ""num_columns"": operation[""p2""],
                ""index_keys"": [],
            })
        elif opcode == ""MakeRecord"":
            registers[operation[""p3""]] = (""MakeRecord"", {
                ""registers"": list(range(operation[""p1""] + operation[""p2""]))
            })
        elif opcode == ""IdxInsert"":
            record = registers[operation[""p2""]]
            cursors[operation[""p1""]][1][""index_keys""].append(record)
        elif opcode == ""Rowid"":
            registers[operation[""p2""]] = (""rowid"", {
                ""table"": operation[""p1""]
            })
        elif opcode == ""Sequence"":
            registers[operation[""p2""]] = (""sequence"", {
                ""next_from_cursor"": operation[""p1""]
            })
        elif opcode == ""Column"":
            registers[operation[""p3""]] = (""column"", {
                ""cursor"": operation[""p1""],
                ""column_offset"": operation[""p2""]
            })
        elif opcode == ""ResultRow"":
            p1 = operation[""p1""]
            p2 = operation[""p2""]
            trace(""ResultRow: "", list(range(p1, p1 + p2)), registers)
            result_row = [registers.get(i) for i in range(p1, p1 + p2)]
        elif opcode == ""Integer"":
            registers[operation[""p2""]] = (""Integer"", operation[""p1""])
        elif opcode == ""String8"":
            registers[operation[""p2""]] = (""String"", operation[""p4""])
        instruction_pointer += 1
    return {""registers"": registers, ""cursors"": cursors, ""result_row"": result_row}
```
Results are promising!
```
execute_operations(db.execute(""explain select 'hello', 55, rowid, * from searchable"").fetchall())

{'registers': {1: ('String', 'hello'),
  2: ('Integer', 55),
  3: ('rowid', {'table': 0}),
  4: ('rowid', {'table': 0}),
  5: ('column', {'cursor': 0, 'column_offset': 1}),
  6: ('column', {'cursor': 0, 'column_offset': 2}),
  7: ('column', {'cursor': 0, 'column_offset': 3})},
 'cursors': {0: ('database_table', {'rootpage': 32, 'connection': 0})},
 'result_row': [('String', 'hello'),
  ('Integer', 55),
  ('rowid', {'table': 0}),
  ('rowid', {'table': 0}),
  ('column', {'cursor': 0, 'column_offset': 1}),
  ('column', {'cursor': 0, 'column_offset': 2}),
  ('column', {'cursor': 0, 'column_offset': 3})]}
```
Here's what happens with a union across three tables:
```
execute_operations(db.execute(f""""""
explain select data as content from binary_data
union
select pk as content from complex_foreign_keys
union
select name as content from facet_cities
""""""}).fetchall())

{'registers': {1: ('column', {'cursor': 4, 'column_offset': 0}),
  2: ('MakeRecord', {'registers': [0, 1, 2, 3]}),
  3: ('column', {'cursor': 0, 'column_offset': 1}),
  4: ('column', {'cursor': 3, 'column_offset': 0})},
 'cursors': {3: ('ephemeral',
   {'num_columns': 1,
    'index_keys': [('MakeRecord', {'registers': [0, 1]}),
     ('MakeRecord', {'registers': [0, 1]}),
     ('MakeRecord', {'registers': [0, 1, 2, 3]})]}),
  2: ('database_table', {'rootpage': 44, 'connection': 0}),
  4: ('database_table', {'rootpage': 24, 'connection': 0}),
  0: ('database_table', {'rootpage': 42, 'connection': 0})},
 'result_row': [('column', {'cursor': 3, 'column_offset': 0})]}
```
Note how the result_row refers to cursor 3, which is an ephemeral table which had three different sets of `MakeRecord` index keys assigned to it - indicating that the output column is NOT from the same underlying table source.","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",849978964,