mirror of https://github.com/sqlite/sqlite
:-) (CVS 106)
FossilOrigin-Name: d825931dea3ef91600ec0b1f95c6c6b094b5bd96
This commit is contained in:
parent
1fd14a78e5
commit
167b16e473
|
@ -188,6 +188,9 @@ fileformat.html: $(TOP)/www/fileformat.tcl
|
|||
lang.html: $(TOP)/www/lang.tcl
|
||||
tclsh $(TOP)/www/lang.tcl >lang.html
|
||||
|
||||
vdbe.html: $(TOP)/www/vdbe.tcl
|
||||
tclsh $(TOP)/www/vdbe.tcl >vdbe.html
|
||||
|
||||
arch.html: $(TOP)/www/arch.tcl
|
||||
tclsh $(TOP)/www/arch.tcl >arch.html
|
||||
|
||||
|
@ -210,6 +213,7 @@ PUBLISH = \
|
|||
opcode.html \
|
||||
arch.html \
|
||||
arch.png \
|
||||
vdbe.html \
|
||||
c_interface.html
|
||||
|
||||
website: $(PUBLISH)
|
||||
|
|
19
manifest
19
manifest
|
@ -1,7 +1,7 @@
|
|||
C :-)\s(CVS\s105)
|
||||
D 2000-06-21T14:00:40
|
||||
C :-)\s(CVS\s106)
|
||||
D 2000-06-23T17:02:08
|
||||
F COPYRIGHT 74a8a6531a42e124df07ab5599aad63870fa0bd4
|
||||
F Makefile.in 4dc16840f68e3b599915e1ec8463d365474dd286
|
||||
F Makefile.in 02ecb0cd0de7ddf7b4623d480061870798787556
|
||||
F README 51f6a4e7408b34afa5bc1c0485f61b6a4efb6958
|
||||
F configure c366a0402bce79ef11fe1bf703ad6ce4ff6afbb0 x
|
||||
F configure.in 1085ff994a334b131325de906ed318e926673588
|
||||
|
@ -59,12 +59,13 @@ F www/arch.png c4d908b79065a72e7dcf19317f36d1324c550e87
|
|||
F www/arch.tcl 4f6a9afecc099a27bba17b4f8cc9561abc15dc40
|
||||
F www/c_interface.tcl 8eb800f67e6896b1894d666b81c0b418cea09fc7
|
||||
F www/changes.tcl 0efac47d38f201e31428dc9f77846049a26e847b
|
||||
F www/fileformat.tcl b11435fcd2cf2238a1c5e6d16fe5e83bcd14d434
|
||||
F www/fileformat.tcl f3a70650e942262f8285d53097d48f0b3aa59862
|
||||
F www/index.tcl 4116afce6a8c63d68882d2b00aa10b079e0129cd
|
||||
F www/lang.tcl 1645e9107d75709be4c6099b643db235bbe0a151
|
||||
F www/opcode.tcl 3cdc4bb2515fcfcbe853e3f0c91cd9199e82dadd
|
||||
F www/sqlite.tcl 9fdcfd48fe9e5669116d02f29b2608903f295740
|
||||
P e1bf96a467b739373191bf75e6a097fc0f24bffc
|
||||
R e797418066fd46003da2b962ee99d71d
|
||||
F www/opcode.tcl 401bdc639509c2f17d3bb97cbbdfdc22a61faa07
|
||||
F www/sqlite.tcl b685dc3ce345a6db0441e6d5716ed29abb96dd29
|
||||
F www/vdbe.tcl 7fe9c3f4358d0c6e27ba8eaa37538a0c1b9b7a05
|
||||
P 516f022206a6000fb2a535c1dbf3e5cd9e077465
|
||||
R 82d5d61965b8834aaa7edc2175faf278
|
||||
U drh
|
||||
Z 9b7592dac834377e51fab9f36928bf0c
|
||||
Z f758ebc92b85fe450c65b87f3da373b0
|
||||
|
|
|
@ -1 +1 @@
|
|||
516f022206a6000fb2a535c1dbf3e5cd9e077465
|
||||
d825931dea3ef91600ec0b1f95c6c6b094b5bd96
|
|
@ -1,7 +1,7 @@
|
|||
#
|
||||
# Run this Tcl script to generate the fileformat.html file.
|
||||
#
|
||||
set rcsid {$Id: fileformat.tcl,v 1.1 2000/06/08 19:38:36 drh Exp $}
|
||||
set rcsid {$Id: fileformat.tcl,v 1.2 2000/06/23 17:02:09 drh Exp $}
|
||||
|
||||
puts {<html>
|
||||
<head>
|
||||
|
@ -27,8 +27,8 @@ proc Code {body} {
|
|||
regsub -all {&} [string trim $body] {\&} body
|
||||
regsub -all {>} $body {\>} body
|
||||
regsub -all {<} $body {\<} body
|
||||
regsub -all {\(\(\(} $body {<font color="#00671f"><i>} body
|
||||
regsub -all {\)\)\)} $body {</i></font>} body
|
||||
regsub -all {\(\(\(} $body {<font color="#00671f"><u>} body
|
||||
regsub -all {\)\)\)} $body {</u></font>} body
|
||||
puts $body
|
||||
puts {</pre></blockquote>}
|
||||
}
|
||||
|
|
|
@ -1,7 +1,7 @@
|
|||
#
|
||||
# Run this Tcl script to generate the sqlite.html file.
|
||||
#
|
||||
set rcsid {$Id: opcode.tcl,v 1.2 2000/06/09 14:14:34 drh Exp $}
|
||||
set rcsid {$Id: opcode.tcl,v 1.3 2000/06/23 17:02:09 drh Exp $}
|
||||
|
||||
puts {<html>
|
||||
<head>
|
||||
|
@ -54,6 +54,14 @@ the statement. The program is generated for a "virtual machine" implemented
|
|||
by the SQLite library. This document describes the operation of that
|
||||
virtual machine.</p>
|
||||
|
||||
<p>This document is intended as a reference, not a tutorial.
|
||||
A separate <a href="vdbe.html">Virtual Machine Tutorial</a> is currently
|
||||
in preparation. If you are looking for a narrative description
|
||||
of how the virtual machine works, you should read the tutorial
|
||||
and not this document. Once you have a basic idea of what the
|
||||
virtual machine does, you can refer back to this document for
|
||||
the details on a particular opcode.</p>
|
||||
|
||||
<p>The source code to the virtual machine is in the <b>vdbe.c</b> source
|
||||
file. All of the opcode definitions further down in this document are
|
||||
contained in comments in the source file. In fact, the opcode table
|
||||
|
@ -156,8 +164,8 @@ proc Code {body} {
|
|||
regsub -all {&} [string trim $body] {\&} body
|
||||
regsub -all {>} $body {\>} body
|
||||
regsub -all {<} $body {\<} body
|
||||
regsub -all {\(\(\(} $body {<font color="#00671f"><i>} body
|
||||
regsub -all {\)\)\)} $body {</i></font>} body
|
||||
regsub -all {\(\(\(} $body {<font color="#00671f"><u>} body
|
||||
regsub -all {\)\)\)} $body {</u></font>} body
|
||||
puts $body
|
||||
puts {</pre></blockquote>}
|
||||
}
|
||||
|
|
|
@ -1,7 +1,7 @@
|
|||
#
|
||||
# Run this Tcl script to generate the sqlite.html file.
|
||||
#
|
||||
set rcsid {$Id: sqlite.tcl,v 1.9 2000/06/21 13:59:14 drh Exp $}
|
||||
set rcsid {$Id: sqlite.tcl,v 1.10 2000/06/23 17:02:09 drh Exp $}
|
||||
|
||||
puts {<html>
|
||||
<head>
|
||||
|
@ -42,8 +42,8 @@ proc Code {body} {
|
|||
regsub -all {&} [string trim $body] {\&} body
|
||||
regsub -all {>} $body {\>} body
|
||||
regsub -all {<} $body {\<} body
|
||||
regsub -all {\(\(\(} $body {<font color="#00671f"><i>} body
|
||||
regsub -all {\)\)\)} $body {</i></font>} body
|
||||
regsub -all {\(\(\(} $body {<font color="#00671f"><u>} body
|
||||
regsub -all {\)\)\)} $body {</u></font>} body
|
||||
puts $body
|
||||
puts {</pre></blockquote>}
|
||||
}
|
||||
|
@ -63,8 +63,8 @@ sqlite>
|
|||
|
||||
puts {
|
||||
<p>(In the example above, and in all subsequent examples, the commands
|
||||
you type are shown with a green tint in an italic font and the responses
|
||||
from the computer are shown in black with a constant-width font.)</p>
|
||||
you type are underlined shown with a green tint and the responses
|
||||
from the computer are shown in black without underlining.)</p>
|
||||
|
||||
<p>You can terminate the sqlite program by typing your systems
|
||||
End-Of-File character (usually a Control-D) or the interrupt
|
||||
|
|
|
@ -0,0 +1,718 @@
|
|||
#
|
||||
# Run this Tcl script to generate the vdbe.html file.
|
||||
#
|
||||
set rcsid {$Id: vdbe.tcl,v 1.1 2000/06/23 17:02:18 drh Exp $}
|
||||
|
||||
puts {<html>
|
||||
<head>
|
||||
<title>The Virtual Database Engine of SQLite</title>
|
||||
</head>
|
||||
<body bgcolor=white>
|
||||
<h1 align=center>
|
||||
The Virtual Database Engine of SQLite
|
||||
</h1>}
|
||||
puts "<p align=center>
|
||||
(This page was last modified on [lrange $rcsid 3 4] GMT)
|
||||
</p>"
|
||||
puts {
|
||||
<blockquote><font color="red"><b>This document is
|
||||
currently under development. It is incomplete and contains
|
||||
errors. Use it accordingly.</b></font></blockquote>
|
||||
}
|
||||
|
||||
puts {
|
||||
<p>If you want to know how the SQLite library works internally,
|
||||
you need to begin with a solid understanding of the Virtual Database
|
||||
Engine or VDBE. The VDBE occurs right in the middle of the
|
||||
processing stream (see the <a href="arch.html">architecture diagram</a>)
|
||||
and so it seems to touch most as parts of the library. Even
|
||||
parts of the code that do not directly interact with the VDBE
|
||||
are usually in a supporting role. The VDBE really is the heart of
|
||||
SQLite.</p>
|
||||
|
||||
<p>This article is a brief tutorial introduction to how the VDBE
|
||||
works and in particular how the various VDBE instructions
|
||||
(documented <a href="opcode.html">here</a>) work together
|
||||
to do useful things with the database. The style is tutorial,
|
||||
beginning with simple tasks and working toward solving more
|
||||
complex problems. Along the way we will touch briefly on most
|
||||
aspects of the SQLite library. After completeing this tutorial,
|
||||
you should have a pretty good understanding of how SQLite works
|
||||
and will be ready to begin studying the actual source code.</p>
|
||||
|
||||
<h2>Preliminaries</h2>
|
||||
|
||||
<p>The VDBE implements a virtual computer that runs a program in
|
||||
its virtual machine language. The goal of each program is to
|
||||
interagate or change the database. Toward this end, the machine
|
||||
language that the VDBE implements is specifically designed to
|
||||
work with databases.</p>
|
||||
|
||||
<p>Each instruction of the VDBE language contains an opcode and
|
||||
three operands labeled P1, P2, and P3. Operand P1 is an arbitrary
|
||||
integer. P2 is a non-negative integer. P3 is a null-terminated
|
||||
string, or possibly just a null pointer. Only a few VDBE
|
||||
instructions use all three operands. Many instructions use only
|
||||
one or two operands. A significant number of instructions use
|
||||
no operands at all, taking there data and storing their results
|
||||
on the execution stack. The details of what each instruction
|
||||
does and which operands it uses are described in the separate
|
||||
<a href="opcode.html">opcode description</a> document.</p>
|
||||
|
||||
<p>A VDBE program begins
|
||||
execution on instruction 0 and continues with successive instructions
|
||||
until it either (1) encounters a fatal error, (2) executes a
|
||||
Halt instruction, or (3) advances the program counter past the
|
||||
last instruction of the program. When the VDBE completes execution,
|
||||
all open database cursors are closed, all memory is freed, and
|
||||
everything is popped from the stack.
|
||||
So there are never any worries about memory leaks or
|
||||
undeallocated resources.</p>
|
||||
|
||||
<p>If you have done any assembly language programming or have
|
||||
worked with any kind of abstract machine before, all of these
|
||||
details should be familiar to you. So let's jump right in and
|
||||
start looking as some code.</p>
|
||||
|
||||
<a name="insert1">
|
||||
<h2>Inserting Records Into The Database</h2>
|
||||
|
||||
<p>We begin with a problem that can be solved using a VDBE program
|
||||
that is only a few instructions long. Suppose we have an SQL
|
||||
table that was created like this:</p>
|
||||
|
||||
<blockquote><pre>
|
||||
CREATE TABLE ex1(col1 text);
|
||||
</pre></blockquote>
|
||||
|
||||
<p>In words, we have a database table named "ex1" that has a single
|
||||
column of data named "col1". Now suppose we want to insert a single
|
||||
record into this table. Like this:</p>
|
||||
|
||||
<blockquote><pre>
|
||||
INSERT INTO ex1 VALUES('Hello, World!');
|
||||
</pre></blockquote>
|
||||
|
||||
<p>We can see the VDBE program that SQLite uses to implement this
|
||||
INSERT using the <b>sqlite</b> command-line utility. First start
|
||||
up <b>sqlite</b> on a new, empty database, then create the table.
|
||||
Finally, enter the INSERT statement shown above, but precede the
|
||||
INSERT with the special keyword "EXPLAIN". The EXPLAIN keyword
|
||||
will cause <b>sqlite</b> to print the VDBE program rather than
|
||||
execute it. We have:</p>
|
||||
}
|
||||
|
||||
proc Code {body} {
|
||||
puts {<blockquote><pre>}
|
||||
regsub -all {&} [string trim $body] {\&} body
|
||||
regsub -all {>} $body {\>} body
|
||||
regsub -all {<} $body {\<} body
|
||||
regsub -all {\(\(\(} $body {<font color="#00671f"><u>} body
|
||||
regsub -all {\)\)\)} $body {</u></font>} body
|
||||
puts $body
|
||||
puts {</pre></blockquote>}
|
||||
}
|
||||
|
||||
Code {
|
||||
$ (((sqlite test_database_1)))
|
||||
sqlite> (((CREATE TABLE ex1(col1 test);)))
|
||||
sqlite> (((.explain)))
|
||||
sqlite> (((EXPLAIN INSERT INTO ex1 VALUES('Hello, World!');)))
|
||||
addr opcode p1 p2 p3
|
||||
---- ------------ ----- ----- ----------------------------------------
|
||||
0 Open 0 1 ex1
|
||||
1 New 0 0
|
||||
2 String 0 0 Hello, World!
|
||||
3 MakeRecord 1 0
|
||||
4 Put 0 0
|
||||
}
|
||||
|
||||
puts {<p>As you can see above, our simple insert statement is
|
||||
implemented in just 5 instructions. There are no jumps, so the
|
||||
program executes once through from top to bottom. Let's now
|
||||
look at each instruction in detail.</p>
|
||||
|
||||
<p>The first instruction opens a cursor that points into the
|
||||
"ex1" table. The P1 operand is a handle for the cursor: zero
|
||||
in this case. Cursor handles can be any non-negative integer.
|
||||
But the VDBE allocates cursors in an array with the size of the
|
||||
array being one more than the largest cursor. So to conserve
|
||||
memory, it is best to use handles beginning with zero and
|
||||
working upward consecutively.</p>
|
||||
|
||||
<p>The P2 operand to the open instruction is 1 which means
|
||||
that the cursor is opened for writing. 0 would have been used
|
||||
for P2 if we wanted to open the cursor for reading only.
|
||||
It is acceptable to open more than one cursor to the same
|
||||
database file at the same time. But all simultaneously
|
||||
opened cursors must be opened with the same P2 value. It is
|
||||
not allowed to have one cursor open for reading a file and
|
||||
another cursor open for writing that same file.</p>
|
||||
|
||||
<p>The second instruction, New, generates an integer key that
|
||||
has not been previously used in the file "ex1". The New instruction
|
||||
uses its P1 operand as the handle of a cursor for the file
|
||||
for which the new key will be generated. The new key is
|
||||
pushed onto the stack. The P2 and P3 operands are not used
|
||||
by the New instruction.</p>
|
||||
|
||||
<p>The third instruction, String, simply pushes its P3
|
||||
operand onto the stack. After the string instruction executes,
|
||||
the stack will contain two elements, as follows:</p>
|
||||
}
|
||||
|
||||
proc stack args {
|
||||
puts "<blockquote><table border=2>"
|
||||
foreach elem $args {
|
||||
puts "<tr><td align=center>$elem</td></tr>"
|
||||
}
|
||||
puts "</table></blockquote>"
|
||||
}
|
||||
|
||||
stack {The string "Hello, World!"} {A random integer key}
|
||||
|
||||
puts {<p>The 4th instructionn, MakeRecord, pops the top P1
|
||||
elements off the stack (1 element in this case) and converts them
|
||||
all into the binary format used for storing records in a
|
||||
database file. (See the <a href="fileformat.html">file format</a>
|
||||
description for details.) The record format consists of
|
||||
a header with one integer for each column giving the offset
|
||||
into the record for the beginning of data for that column.
|
||||
Following the header is the data for each column, Each column
|
||||
is stored as a null-terminated ASCII text string. The new
|
||||
record generated by the MakeRecord instruction is pushed back
|
||||
onto the stack, so that after the 4th instruction executes,
|
||||
the stack looks like this:</p>
|
||||
}
|
||||
|
||||
stack {A one-column record containing "Hello, World!"} \
|
||||
{A random integer key}
|
||||
|
||||
puts {<p>The last instruction pops top elements from the stack
|
||||
and uses them as data and key to make a new entry in database
|
||||
database file pointed to by cursor P1. This instruction is where
|
||||
the insert actually occurs.</p>
|
||||
|
||||
<p>After the last instruction executes, the program counter
|
||||
advances to one past the last instruction, which causes the
|
||||
VDBE to halt. When the VDBE halts, it automatically closes
|
||||
all open cursors, frees any elements left on the stack,
|
||||
and releases any other resources we may have allocated.
|
||||
In this case, the only cleanup necessary is to close the
|
||||
open cursor to the "ex1" file.</p>
|
||||
|
||||
<a name="trace">
|
||||
<h2>Tracing VDBE Program Execution</h2>
|
||||
|
||||
<p>If the SQLite library is compiled without the NDEBUG
|
||||
preprocessor macro being defined, then
|
||||
there is a special SQL comment that will cause the
|
||||
the VDBE to traces the execution of programs.
|
||||
Though this features was originally intended for testing
|
||||
and debugging, it might also be useful in learning about
|
||||
how the VDBE operates.
|
||||
Use the "<tt>--vdbe-trace-on--</tt>" comment to
|
||||
turn tracing on and "<tt>--vdbe-trace-off--</tt>" to turn tracing
|
||||
back off. Like this:</p>
|
||||
}
|
||||
|
||||
Code {
|
||||
sqlite> (((--vdbe-trace-on--)))
|
||||
...> (((INSERT INTO ex1 VALUES('Hello, World!');)))
|
||||
0 Open 0 1 ex1
|
||||
1 New 0 0
|
||||
Stack: i:179007474
|
||||
2 String 0 0 Hello, World!
|
||||
Stack: s:[Hello, Worl] i:179007474
|
||||
3 MakeRecord 1 0
|
||||
Stack: z:[] i:179007474
|
||||
4 Put 0 0
|
||||
}
|
||||
|
||||
puts {
|
||||
<p>With tracing mode on, the VDBE prints each instruction prior
|
||||
to executing it. After the instruction is executed, the top few
|
||||
entries in the stack are displayed. The stack display is omitted
|
||||
if the stack is empty.</p>
|
||||
|
||||
<p>On the stack display, most entries are show with a prefix
|
||||
that tells the datatype of that stack entry. Integers begin
|
||||
with "<tt>i:</tt>". Floating point values begin with "<tt>r:</tt>".
|
||||
(The "r" stands for "real-number".) Strings begin with either
|
||||
"<tt>s:</tt>" or "<tt>z:</tt>". The difference between s: and
|
||||
z: strings is that z: strings are stored in memory obtained
|
||||
from <b>malloc()</b>. This doesn't make any difference to you,
|
||||
the observer, but it is vitally important to the VDBE since the
|
||||
z: strings need to be passed to <b>free()</b> when they are
|
||||
popped to avoid a memory leak. Note that only the first 10
|
||||
characters of string values are displayed and that binary
|
||||
values (such as the result of the MakeRecord instruction) are
|
||||
treated as strings. The only other datatype that can be stored
|
||||
on the VDBE stack is a NULL, which is display without prefix
|
||||
as simply "<tt>NULL</tt>".
|
||||
|
||||
<a name="query1">
|
||||
<h2>Simple Queries</h2>
|
||||
|
||||
<p>At this point, you should understand the basics of how the VDBE
|
||||
writes to a database. Now let's look at how it does queries.
|
||||
We will use the follow simple SELECT statement as our example:</p>
|
||||
|
||||
<blockquote><pre>
|
||||
SELECT col1 FROM ex1;
|
||||
</pre></blockquote>
|
||||
|
||||
<p>The VDBE program generated for this SQL statement is as follows:</p>
|
||||
}
|
||||
|
||||
Code {
|
||||
sqlite> (((EXPLAIN SELECT * FROM ex1;)))
|
||||
addr opcode p1 p2 p3
|
||||
---- ------------ ----- ----- ----------------------------------------
|
||||
0 ColumnCount 1 0
|
||||
1 ColumnName 0 0 col1
|
||||
2 Open 0 0 ex1
|
||||
3 Next 0 7
|
||||
4 Field 0 0
|
||||
5 Callback 1 0
|
||||
6 Goto 0 3
|
||||
7 Close 0 0
|
||||
}
|
||||
|
||||
puts {
|
||||
<p>Before we begin looking at this problem, let's briefly review
|
||||
how queries work in SQLite so that we will know what we are trying
|
||||
to accomplish. For each row in the result of a query,
|
||||
SQLite will invoke a callback function with the following
|
||||
prototype:</p>
|
||||
|
||||
<blockquote><pre>
|
||||
int Callback(void *pUserData, int nColumn, char *azData[], char *azColumnName[]);
|
||||
</pre></blockquote>
|
||||
|
||||
<p>The SQLite library supplies the VDBE with a pointer to the callback function
|
||||
itself, and the <b>pUserData</b> pointer. The job of the VDBE is to
|
||||
come up with values for <b>nColumn</b>, <b>azData[]</b>,
|
||||
and <b>azColumnName[]</b>.
|
||||
<b>nColumn</b> is the number of columns in the results, of course.
|
||||
<b>azColumnName[]</b> is an array of strings where each string is the name
|
||||
of one of the result column. <b>azData[]</b> is an array of strings holding
|
||||
the actual data.</p>
|
||||
|
||||
<p>The first two instructions in the VDBE program for our query are
|
||||
considered with setting up values for <b>azColumn</b>.
|
||||
The ColumnCount instruction tells the VDBE how much space to allocate
|
||||
for the <b>azColumnName[]</b> array. The ColumnName instructions
|
||||
tell the VDBE what value to fill in for each element of the
|
||||
<b>azColumnName[]</b> array. Every query will begin with once
|
||||
ColumnCount instruction and once ColumnName instruction for each
|
||||
column in the result.</p>
|
||||
|
||||
<p>The third instruction opens a cursor into the database file
|
||||
that is to be queried. This works the same as the Open instruction
|
||||
in the INSERT example <a href="#insert1">above</a> except that the
|
||||
cursor is opened for reading this time instead of for writing.</p>
|
||||
|
||||
<p>The instructions at address 3 and 6 form a loop that will execute
|
||||
once for each record in the database file. This is a key concept that
|
||||
you should pay close attention to. The Next instruction at
|
||||
address 3 tell the VDBE to advance the cursor (identified by P1)
|
||||
to the next record. The first time Next instruction is executed,
|
||||
the cursor is set to the first record of the file. If there are
|
||||
no more records in the database file when Next is executed, then
|
||||
the VDBE makes an immediate jump over the body of the loop to
|
||||
instruction 7 (specified by operand P2). The body of the loop
|
||||
is formed by instructions at addresses 4 and 5. After the loop
|
||||
body is an unconditional jump at instruction 6 which takes us
|
||||
back to the Next instruction at the beginning of the loop.
|
||||
</p>
|
||||
|
||||
<p>The body of the loop consists of instructions at addresses 4 and
|
||||
5. The Field instruction at address 4 takes the P2-th column from
|
||||
the P1-th cursor and pushes it onto the stack.
|
||||
(The "Field" instruction probably should be renamed as the "Column"
|
||||
instruction.) In this example, the Field instruction is pushing the
|
||||
value for the "col1" data column onto the stack.</p>
|
||||
|
||||
<p>The Callback instruction at address 5 invokes the callback function.
|
||||
The P1 operand to callback becomes the value for <b>nColumn</b>.
|
||||
The Callback instruction also pops P1 values from the stack and
|
||||
uses them to form the <b>azData[]</b> for the callback.</p>
|
||||
|
||||
<p>The Close instruction at the end of the program closes the
|
||||
cursor that points into the database file. It is not really necessary
|
||||
to call close here since all cursors will be automatically closed
|
||||
by the VDBE when the program halts. But we needed an instruction
|
||||
for the Next to jump to so we might as well go ahead and have that
|
||||
instruction do something useful.</p>
|
||||
|
||||
<a name="query2">
|
||||
<h2>A Slightly More Complex Query</h2>
|
||||
|
||||
<p>The key points of the previous example where the use of the Callback
|
||||
instruction to invoke the callback function, and the use of the Next
|
||||
instruction to implement a loop over all records of the database file.
|
||||
This example attempts to drive home those ideas by demonstrating a
|
||||
slightly more complex query that involves multiple columns of
|
||||
output, some of which are computed values, and a WHERE clause that
|
||||
limits which records actually make it to the callback function.
|
||||
Consider this query:</p>
|
||||
|
||||
<blockquote><pre>
|
||||
SELECT col1 AS 'Name', '**' || col1 || '**' AS 'With Stars'
|
||||
FROM ex1
|
||||
WHERE col1 LIKE '%ll%'
|
||||
</pre></blockquote>
|
||||
|
||||
<p>This query is perhaps a bit contrived, but it does serve to
|
||||
illustrate our points. The result will have two column with
|
||||
names "Name" and "With Stars". The first column is just the
|
||||
sole column in our simple example table. The second column
|
||||
of the result is the same as the first column except that
|
||||
asterisks have been prepended and appended. Finally, the
|
||||
WHERE clause says that we will only chose rows for the
|
||||
results that contain two "l" characters in a row. Here is
|
||||
what the VDBE program looks like for this query:</p>
|
||||
}
|
||||
|
||||
Code {
|
||||
sqlite> (((EXPLAIN SELECT col1 AS 'Name', '**' || col1 || '**' AS 'With Stars')))
|
||||
...> (((FROM ex1)))
|
||||
...> (((WHERE col1 LIKE '%ll%';)))
|
||||
addr opcode p1 p2 p3
|
||||
---- ------------ ----- ----- ----------------------------------------
|
||||
0 ColumnCount 2 0
|
||||
1 ColumnName 0 0 Name
|
||||
2 ColumnName 1 0 With Stars
|
||||
3 Open 0 0 ex1
|
||||
4 Next 0 16
|
||||
5 Field 0 0
|
||||
6 String 0 0 %ll%
|
||||
7 Like 1 4
|
||||
8 Field 0 0
|
||||
9 String 0 0 **
|
||||
10 Field 0 0
|
||||
11 Concat 2 0
|
||||
12 String 0 0 **
|
||||
13 Concat 2 0
|
||||
14 Callback 2 0
|
||||
15 Goto 0 4
|
||||
16 Close 0 0
|
||||
}
|
||||
|
||||
puts {
|
||||
<p>Except for the WHERE clause, the structure of the program for
|
||||
this example is very much like the prior example, just with an
|
||||
extra column. The ColumnCount is 2 now, instead of 1 as before,
|
||||
and there are two ColumnName instructions.
|
||||
A cursor is opened using the Open instruction, just like in the
|
||||
prior example. The Next instruction at address 4 and the
|
||||
Goto at address 15 form a loop over all records of the database
|
||||
file. The Close instruction at the end is there to give the
|
||||
Next instruction something to jump to when it is done. All of
|
||||
this is just like in the first query demonstration.</p>
|
||||
|
||||
<p>The Callback instruction in this example has to generate
|
||||
data for two result columns instead of one, but is otherwise
|
||||
the same as in the first query. When the Callback instruction
|
||||
is invoked, the left-most column of the result should be
|
||||
the lowest in the stack and the right-most result column should
|
||||
be the top of the stack. We can see the stack being set up
|
||||
this way at addresses 8 through 13. The Field instruction at
|
||||
8 pushes the value of the "col1" column of table "ex1" onto the
|
||||
stack, and that is all that has to be done for the left column
|
||||
of the result. Instructions at 9 through 13 evaluate the
|
||||
expression used for the second result column and leave it
|
||||
on the stack as well.</p>
|
||||
|
||||
<p>The only thing that is really new about the current example
|
||||
is the WHERE clause which is implemented by instructions at
|
||||
addresses 5, 6, and 7. Instructions at address 5 and 6 push
|
||||
onto the stack the value of the "col1" column and the literal
|
||||
string "%ll%". The Like instruction at address 7 pops these
|
||||
two values from the stack and causes an
|
||||
immediate jump back to the Next instruction if the "col1" value
|
||||
is <em>not</em> like the literal string "%ll%". Taking this
|
||||
jump effectively skips the callback, which is the whole point
|
||||
of the WHERE clause. If the result
|
||||
of the comparison is true, the jump is not taken and control
|
||||
falls through to the Callback instruction below.</p>
|
||||
|
||||
<p>Notice how the Like instruction works. It uses the top of
|
||||
the stack as its pattern and the next on stack as the data
|
||||
to compare. Because P1 is 1, a jump is made to P2 if the
|
||||
comparison fails. So with P1 equal to one, a more precise
|
||||
name for this instruction might be "Jump If NOS Is Not Like TOS".
|
||||
The sense of the test in inverted if P1 is 0. So when P1
|
||||
is zero, the instruction is more like "Jump If NOS Is Like TOS".
|
||||
</p>
|
||||
|
||||
<a name="pattern1">
|
||||
<h2>A Template For SELECT Programs</h2>
|
||||
|
||||
<p>The first two query examples illustrate a kind of template that
|
||||
every SELECT program will follow. Basically, we have:</p>
|
||||
|
||||
<p>
|
||||
<ol>
|
||||
<li>Initialize the <b>azColumnName[]</b> array for the callback.</li>
|
||||
<li>Open a cursor into the table to be queried.</li>
|
||||
<li>For each record in the table, do:
|
||||
<ol type="a">
|
||||
<li>If the WHERE clause evaluates to FALSE, then skip the steps that
|
||||
follow and continue to the next record.</li>
|
||||
<li>Compute all columns for the current row of the result.</li>
|
||||
<li>Invoke the callback function for the current row of the result.</li>
|
||||
</ol>
|
||||
<li>Close the cursor.</li>
|
||||
</ol>
|
||||
</p>
|
||||
|
||||
<p>This template will be expanded considerably as we consider
|
||||
additional complications such as joins, compound selects, using
|
||||
indices to speed the search, sorting, and aggregate functions
|
||||
with and without GROUP BY and HAVING clauses.
|
||||
But the same basic ideas will continue to apply.</p>
|
||||
|
||||
<h2>UPDATE And DELETE Statements</h2>
|
||||
|
||||
<p>The UPDATE and DELETE statements are coded using a template
|
||||
that is very similar to the SELECT statement template. The main
|
||||
difference, of course, is that the end action is to modify the
|
||||
database rather than invoke a callback function. Let's begin
|
||||
by looking at a DELETE statement:</p>
|
||||
|
||||
<blockquote><pre>
|
||||
DELETE FROM ex1 WHERE col1 NOT LIKE '%H%'
|
||||
</pre></blockquote>
|
||||
|
||||
<p>This DELETE statement will remove every record from the "ex1"
|
||||
table that does not contain an "H" characters in the "col1"
|
||||
column. The code generated to do this is as follows:</p>
|
||||
}
|
||||
|
||||
Code {
|
||||
sqlite> (((EXPLAIN DELETE FROM ex1 WHERE col1 NOT LIKE '%H%';)))
|
||||
addr opcode p1 p2 p3
|
||||
---- ------------ ----- ----- ----------------------------------------
|
||||
0 ListOpen 0 0
|
||||
1 Open 0 0 ex1
|
||||
2 Next 0 9
|
||||
3 Field 0 0
|
||||
4 String 0 0 %H%
|
||||
5 Like 0 2
|
||||
6 Key 0 0
|
||||
7 ListWrite 0 0
|
||||
8 Goto 0 2
|
||||
9 Close 0 0
|
||||
10 ListRewind 0 0
|
||||
11 Open 0 1 ex1
|
||||
12 ListRead 0 15
|
||||
13 Delete 0 0
|
||||
14 Goto 0 12
|
||||
15 ListClose 0 0
|
||||
}
|
||||
|
||||
puts {
|
||||
<p>Here is what the program must do. First it has to locate all of
|
||||
the records in the "ex1" database that are to be deleted. This is
|
||||
done using a loop very much like the loop used in the SELECT examples
|
||||
above. Once all records have been located, then we can go back through
|
||||
an delete them one by one. Note that we cannot delete each record
|
||||
as soon as we find it. We have to locate all records first, then
|
||||
go back and delete them. This is because with the GDBM database
|
||||
backend (as with most other backends based on hashing) when you
|
||||
delete a record it might change the scan order. And if the scan
|
||||
order changes in the middle of the scan, some records might be
|
||||
tested more than once, and some records might not be tested at all.</p>
|
||||
|
||||
<p>So the implemention of DELETE is really in two loops. The
|
||||
first loop (instructions 3 through 8 in the example) locates the records that
|
||||
are to be deleted and the second loop (instructions 12 through 14)
|
||||
do the actual deleting.</p>
|
||||
|
||||
<p>The very first instruction in the program, the ListOpen instruction,
|
||||
creates a new List object in which we can store the keys of the records
|
||||
that are to be deleted. The P1 operand serves as a handle to the
|
||||
list. As with cursors, you can open as many lists as you like
|
||||
(though in practice we never need more than one at a time.) Each list
|
||||
has a handle specified by P1 which is a non-negative integer. The
|
||||
VDBE allocates an array of handles, so it is best to use only small
|
||||
handles. As currently implemented, SQLite never uses more than one
|
||||
list at a time and so it always uses the handle of 0 for every list.</p>
|
||||
|
||||
<p>Each list is really a file descriptor for a temporary file that
|
||||
is created for holding the list. What's going to happen is this: the
|
||||
first loop of the program is going to locate records that need to
|
||||
be deleted and write their keys onto the list. Then the second
|
||||
loop is going to playback the list and delete the records one by one.</p>
|
||||
|
||||
<p>The second instruction opens a cursor to the database file "ex1".
|
||||
Notice that the cursor is opened for reading, not writing. At this
|
||||
stage of the program we are going to be scanning the file not changing
|
||||
it. We will reopen the same file for writing it later, at instruction 11.
|
||||
</p>
|
||||
|
||||
<p>Following the Open, there is a loop composed of the Next instruction
|
||||
at address 2 and continuing down to the Goto at 8. This loop works
|
||||
the same way as the query loops worked in the prior examples. But
|
||||
instead of invoking a callback at the end of each loop iteration, this
|
||||
program calls ListWrite at instruction 7. The ListWrite instruction
|
||||
pops an integer from the stack and appends it to the List identified
|
||||
by P1. The integer is a key to a record that should be deleted and
|
||||
was placed on the stack by the preceding Key instruction.
|
||||
The WHERE clause is implemented by instructions 3, 4, and 5.
|
||||
The implementation of the WHERE clause is exactly the same as in the
|
||||
previous SELECT statement, except that the P1 operand to the Like
|
||||
instruction is 0 instead of one because the DELETE statement uses
|
||||
the NOT LIKE operator instead of the LIKE operator. If the WHERE
|
||||
clause evaluates to false (if col2 is like "%ll%") then the ListWrite
|
||||
instruction gets skipped and the key to that record is never written
|
||||
to the list. Hence, the record is not deleted.</p>
|
||||
|
||||
<p>At the end of the first loop, the cursor is closed at instruction 9,
|
||||
and the list is rewound back to the beginning at instruction 10.
|
||||
Next, instruction 11 reopens the same database file, but for
|
||||
writing this time. The loop that does the actual deleting of records
|
||||
is on instructions 12, 13, and 14.</p>
|
||||
|
||||
<p>The ListRead instruction as 12 reads a single integer key from
|
||||
the list and pushes that key onto the stack. If there are no
|
||||
more keys, nothing gets pushed onto the stack but instead a jump
|
||||
is made to instruction 15. Notice the similarity of operation
|
||||
between the ListRead and Next instructions. Both operations work
|
||||
something like this:</p>
|
||||
|
||||
<blockquote>
|
||||
Push the next "thing" onto the stack and fall through.
|
||||
Or if there is no next "thing" to push, jump immediately to P2.
|
||||
</blockquote>
|
||||
|
||||
<p>The only difference between Next and ListRead is the definition
|
||||
of "next thing". The "things" for the Next instruction are records
|
||||
in a database file. "Things" for ListRead are integer keys in a list.
|
||||
Later on,
|
||||
we will see other looping instructions (NextIdx and SortNext) that
|
||||
operating using the same principle.</p>
|
||||
|
||||
<p>The Delete instruction at address 13 pops an integer key from
|
||||
the stack (the key was put there by the preceding ListRead
|
||||
instruction) and deletes the record of cursor P1 that has that key.
|
||||
If there is not record in the database with the given key, then
|
||||
Delete is a no-op.</p>
|
||||
|
||||
<p>There is a Goto instruction at 14 to complete the second loop.
|
||||
Then at instruction 15 is as ListClose operation. The ListClose
|
||||
closes the list and deletes the temporary that held the ist.
|
||||
Calling ListClose is optional. The VDBE will automatically close
|
||||
the list when it halts. But we need an instruction for the
|
||||
ListRead to jump to when it reaches the end of the list and
|
||||
ListClose seemed like a natural candidate.</p>
|
||||
|
||||
<p>UPDATE statements work very much like DELETE statements except
|
||||
that instead of deleting the record they replace it with a new one.
|
||||
Consider this example:
|
||||
</p>
|
||||
|
||||
<blockquote><pre>
|
||||
UPDATE ex1 SET col1='H' || col1 WHERE col1 NOT LIKE '%H%'
|
||||
</pre></blockquote>
|
||||
|
||||
<p>Instead of deleting records that lack an "H" in column "col1",
|
||||
this statement changes the column by prepending an "H".
|
||||
The VDBE program to implement this statement follows:</p>
|
||||
}
|
||||
|
||||
Code {
|
||||
addr opcode p1 p2 p3
|
||||
---- ------------ ----- ----- ----------------------------------------
|
||||
0 ListOpen 0 0
|
||||
1 Open 0 0 ex1
|
||||
2 Next 0 9
|
||||
3 Field 0 0
|
||||
4 String 0 0 %H%
|
||||
5 Like 0 2
|
||||
6 Key 0 0
|
||||
7 ListWrite 0 0
|
||||
8 Goto 0 2
|
||||
9 Close 0 0
|
||||
10 ListRewind 0 0
|
||||
11 Open 0 1 ex1
|
||||
12 ListRead 0 21
|
||||
13 Dup 0 0
|
||||
14 Fetch 0 0
|
||||
15 String 0 0 H
|
||||
16 Field 0 0
|
||||
17 Concat 2 0
|
||||
18 MakeRecord 1 0
|
||||
19 Put 0 0
|
||||
20 Goto 0 12
|
||||
21 ListClose 0 0
|
||||
}
|
||||
|
||||
puts {
|
||||
<p>This program is exactly the same as the DELETE program
|
||||
except that the single Delete instruction in the second loop
|
||||
has been replace by a sequence of instructions (at addresses
|
||||
13 through 19) that update the record rather than delete it.
|
||||
Most of this instruction sequence you already be familiar to
|
||||
you, but there are a couple of minor twists so we will go
|
||||
over it briefly.</p>
|
||||
|
||||
<p>As we enter the interior of the second loop (at instruction 13)
|
||||
the stack contains a single integer which is the key of the
|
||||
record we want to modify. We are going to need to use this
|
||||
key twice: once to fetch the old value of column "col1" and
|
||||
a second time to write back the new value. So the first instruction
|
||||
is a Dup to make a duplicate of the top of the stack. The
|
||||
VDBE Dup instruction is actually a little more general than that.
|
||||
It will duplicate any element of the stack, not just the top
|
||||
element. You specify which element to duplication using the
|
||||
P1 operand. When P1 is 0, the top of the stack is duplicated.
|
||||
When P1 is 1, the next element down on the stack duplication.
|
||||
And so forth.</p>
|
||||
|
||||
<p>After duplicating the key, the next instruction is Fetch
|
||||
pops the stack once and uses the value popped as a key to
|
||||
load a record from the database file. In this way, we obtain
|
||||
the old column values for the record that is about to be
|
||||
updated.</p>
|
||||
|
||||
<p>Instructions 15 through 18 construct a new database record
|
||||
that will be used to replace the existing record. This is
|
||||
the same kind of code that we say <a href="#insert1">above</a>
|
||||
in the description of INSERT and will not be described further.
|
||||
After instruction 18 executes, the stack looks like this:</p>
|
||||
}
|
||||
|
||||
stack {New data record} {Integer key}
|
||||
|
||||
puts {
|
||||
<p>The Put instruction (also described <a href="#insert1">above</a>
|
||||
during the discussion about INSERT) writes an entry into the
|
||||
database file whose data is the top of the stack and whose key
|
||||
is the next on the stack, and then pops the stack twice. The
|
||||
Put instruction will overwrite the data of an existing record
|
||||
with the same key, which is what we want here. It was not
|
||||
an issue with INSERT because with INSERT the key was generated
|
||||
by the Key instruction which is guaranteed to generate a key
|
||||
that has not been used before. (By the way, since keys must
|
||||
all be unique and each key is a 32-bit integer, a single
|
||||
SQLite database table can have no more than 2<sup>32</sup>
|
||||
rows. Actually, the Key instruction starts to become
|
||||
very inefficient as you approach this upper bound, so it
|
||||
is best to keep the number of entries below 2<sup>31</sup>
|
||||
or so. Surely a couple billion records will be enough for
|
||||
most applications!)</p>
|
||||
|
||||
<p>The rest of the UPDATE program is the same as for DELETE,
|
||||
and for all the same reasons.</p>
|
||||
}
|
||||
|
||||
puts {
|
||||
<p><hr /></p>
|
||||
<p><a href="index.html"><img src="/goback.jpg" border=0 />
|
||||
Back to the SQLite Home Page</a>
|
||||
</p>
|
||||
|
||||
</body></html>}
|
Loading…
Reference in New Issue