Documentation Source Text

<h1>Introduction</h1>

<p>SQLite operates by translating each SQL statement into bytecode.
A [prepared statement] in SQLite is mostly just the bytecode needed to
implement the corresponding SQL.  The [sqlite3_prepare_v2()] interface
is a compiler that translates SQL into bytecode.
The [sqlite3_step()] interface passes that bytecode to a virtual machine.
The virtual machine evaluates the bytecode and thereby does the work
specified by the original SQL statement.
The virtual machine is the heart of SQLite.
Programmers who want to understand how SQLite operates internally
must be familiar with the bytecode engine.

<p>Historically, the bytecode engine in SQLite is called the
"Virtual DataBase Engine" or "VDBE".  This article uses the terms
"bytecode engine" and "VDBE" and "virtual machine" interchangeably.

<p>
This article also uses the terms "bytecode program" and
"prepared statement" interchangeably, as theyalso mean the same thing.

<h2>VDBE Source Code</h2>

<p>The source code to the bytecode engine is in the 
[http://www.sqlite.org/src/finfo?name=src/vdbe.c | vdbe.c] source
file.  All of the [opcode definitions] in this document are
contained in comments in the source file.  In fact, the opcode table

13    TableLock      0     2     1     tbl1           00  iDb=0 root=2 write=1
14    Integer        20    3     0                    00  r[3]=20      
15    Goto           0     1     0                    00               
}
</tcl>

<p>Any application can run an [EXPLAIN] query to get output similar to 
the above.
However, indentation to show the loop structure is not generated
by the SQLite core.  The [command-line shell] contains extra logic
for indenting loops.
Also, the "comment" column in the [EXPLAIN] output
is only provided if SQLite is compiled with the
[-DSQLITE_ENABLE_EXPLAIN_COMMENTS] options.