You start by presenting us with a stand-alone script that we can run to see
your question. See you are getting free help, you really should be
striving to make it easy for people to help you. We don't have a script
from you, so I made one up. I'm using:
CREATE TABLE employee(eid INTEGER PRIMARY KEY, name TEXT);
CREATE TABLE location(lid INTEGER PRIMARY KEY, name TEXT);
CREATE TABLE company(cid INTEGER PRIMARY KEY, name TEXT);
.explain
EXPLAIN
SELECT *
FROM employee, company, location
WHERE location.name=company.name
AND location.name=employee.name;
Be sure to indent your code clearly so that people and understand it more
easily.
For the trunk of SQLite, the first few instructions are:
addr opcode p1 p2 p3 p4 p5 comment
---- ------------- ---- ---- ---- ------------- -- -------------
0 Init 0 44 0 00 Start at 44
1 OpenRead 0 2 0 2 00 root=2 iDb=0;
employee
2 OpenRead 2 3 0 2 00 root=3 iDb=0;
location
3 OpenRead 1 4 0 2 00 root=4 iDb=0;
company
4 Explain 0 0 0 SCAN TABLE employee
00
5 Rewind 0 42 0 00
6 Once 0 14 0 00
7 OpenAutoindex 3 2 0 k(2,nil,nil) 00 nColumn=2; for
location
8 Rewind 2 14 0 00
9 Column 2 1 2 00 r[2]=
location.name
10 Rowid 2 3 0 00 r[3]=rowid
11 MakeRecord 2 2 1 00
r[1]=mkrec(r[2..3])
12 IdxInsert 3 1 0 10 key=r[1]
13 Next 2 9 0 03
I think you are interested in knowing where the P3 operand (the output
register number) for instruction 9 comes from. (Note that the program
shown above might be slightly different in whatever version of SQLite you
are running.)
The way you find this out is to first compile sqlite3 using
-DSQLITE_DEBUG. Then add the statement:
PRAGMA vdbe_addoptrace=ON;
Right before the "EXPLAIN" in the script. That pragma (only available when
SQLite is compiled with -DSQLITE_DEBUG) causes SQLite to print a message on
the screen every time it generates a new opcode. Next, run sqlite3 in a
debugger and set a breakpoint on the "test_addop_breakpoint" procedure.
(This is a dummy procedure created specifically to give you a place to set
a breakpoint.) Then run your script. Continue past the first few
breakpoints until the "Column" instruction is emitted. Now you can examine
the call stack to figure out exactly how that opcode was generated.
It appears that the output register 2 comes from here:
http://www.sqlite.org/src/artifact/fae81cc2eb14b?ln=812-813
Specifically the last parameter. regBase+j. You can continue to follow
the stack to figure out where regBase was computed. There are comments on
each function that try to explain what that particular function is doing.
--
D. Richard Hipp
d...@sqlite.org
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