#
# Run this Tcl script to generate the vdbe.html file.
#
-set rcsid {$Id: vdbe.tcl,v 1.2 2000/06/23 19:16:23 drh Exp $}
+set rcsid {$Id: vdbe.tcl,v 1.3 2000/06/26 12:02:51 drh Exp $}
puts {<html>
<head>
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
+and so it seems to touch most 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
+<p>This article is a brief 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,
+complex problems. Along the way we will visit most
+submodules in 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>
<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
+interrogate or change the database. Toward this end, the machine
language that the VDBE implements is specifically designed to
-work with databases.</p>
+search, read, and modify 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
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
+no operands at all but instead take their 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>
table that was created like this:</p>
<blockquote><pre>
-CREATE TABLE ex(one text, two int);
+CREATE TABLE examp(one text, two int);
</pre></blockquote>
-<p>In words, we have a database table named "ex" that has two
+<p>In words, we have a database table named "examp" that has two
columns of data named "one" and "two". Now suppose we want to insert a single
record into this table. Like this:</p>
<blockquote><pre>
-INSERT INTO ex VALUES('Hello, World!',99);
+INSERT INTO examp VALUES('Hello, World!',99);
</pre></blockquote>
<p>We can see the VDBE program that SQLite uses to implement this
stack {A data record holding "Hello, World!" and 99} \
{A random integer key}
-puts {<p>The last instruction pops top elements from the stack
+puts {<p>The last instruction pops the top two 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>
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 "examp" file.</p>
+cursor to the "examp" 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
+preprocessor macro, 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
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
+<p>On the stack display, most entries are shown 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
the records in the "examp" 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
+and 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 the GDBM database
backend might change the scan order after a delete operation.
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>
+visited more than once and other records might not be visited at all.</p>
<p>So the implemention of DELETE is really in two loops. The
first loop (instructions 2 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>
+does 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
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
+<p>The ListRead instruction at 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
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>
+operate 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
</pre></blockquote>
<p>Instead of deleting records where the "two" column is less than
-50, this statement just puts the "one" column in paraentheses
+50, this statement just puts the "one" column in parentheses
The VDBE program to implement this statement follows:</p>
}
projects / thirdparty / sqlite.git / commitdiff
