Documentation Source Text

For command-line usage, add the short style command-line options immediately
following the "-A", without an intervening space.  All subsequent arguments
are considered to be part of the .archive command.  For example, the following 
commands are equivalent:

<codeblock>
sqlite3 new_archive.db -Acv file1 file2 file3
sqlite3 new_archive.db ".ar -cv file1 file2 file3"
</codeblock>

<p>
Long and short style options may be mixed. For example, the following are
equivalent:

<codeblock>

<title>Measuring and Reducing CPU Usage in SQLite</title>
<tcl>hd_keywords {CPU cycles used} {CPU performance measurement}</tcl>

<table_of_contents>

<h1>Overview</h1>

<p>The graph below shows the number of CPU cycles used by SQLite on a
standard workload, for all versions of SQLite going back about 9 years.
Recent versions of SQLite use less than a third of the CPU cycles 
compared to older versions.

<p>
This article describes how the SQLite developers measure CPU usage,
what those measurements actually mean, and the techniques used by
SQLite developers on their continuing quest to further reduce the
CPU usage of the SQLite library.

<p>Beginning with SQLite [version 3.10.0] ([dateof:3.10.0]), 
the unix OS interface will
attempt to resolve symbolic links and open the database file by its
canonical name.  Prior to version 3.10.0, opening a database file 
through a symbolic link was similar to opening a database file
that had multiple hard links and resulted in undefined behavior.</p>

<tcl>hd_fragment fork</tcl>
<h2> Carrying an open database connection across a fork() </h2>

<p> Do not open an SQLite database connection, then fork(), then try
to use that database connection in the child process.  All kinds of
locking problems will result and you can easily end up with a corrupt
database.  SQLite is not designed to support that kind of behavior.
Any database connection that is used in a child process must be opened
in the child process, not inherited from the parent.

<p> Do not even call [sqlite3_close()] on a database connection from a
child process if the connection was opened in the parent.  It is safe
to close the underlying file descriptor, but the [sqlite3_close()]
interface might invoke cleanup activities that will delete content out
from under the parent, leading to errors and perhaps even database
corruption.

<h1> Failure to sync</h1>

<p>In order to guarantee that database files are always consistent, SQLite
will occasionally ask the operating system to flush all pending writes to
persistent storage then wait for that flush to complete.  This is 
accomplished using the <tt>fsync()</tt> system call under unix and

limited to the value set by
[sqlite3_limit]([SQLITE_LIMIT_COLUMN],...).)^</p>

<tcl>hd_fragment uniqueidx {unique index}</tcl>
<p>^If the UNIQUE keyword appears between CREATE and INDEX then duplicate
index entries are not allowed.  ^Any attempt to insert a duplicate entry
will result in an error.  ^For the purposes of unique indices, all NULL values
are considered different from all other NULL values and are thus unique.
This is one of the two possible interpretations of the SQL-92 standard
(the language in the standard is ambiguous) and is the interpretation
followed by PostgreSQL, MySQL, Firebird, and Oracle.  Informix and
Microsoft SQL Server follow the other interpretation of the standard.</p>

<p>^If the optional IF NOT EXISTS clause is present and another index
with the same name already exists, then this command becomes a no-op.</p>

<center><table border=1>
  <tr><th>Expression Affinity   <th>Column Declared Type
  <tr><td>TEXT                  <td>"TEXT"
  <tr><td>NUMERIC               <td>"NUM"
  <tr><td>INTEGER               <td>"INT"
  <tr><td>REAL                  <td>"REAL"
  <tr><td>BLOB (a.k.a "NONE")   <td>"" (empty string)
</table></center>)^

<p>^(A table created using CREATE TABLE AS has no PRIMARY KEY and no
constraints of any kind. The default value of each column is NULL. The default
collation sequence for each column of the new table is BINARY.)^

<p>^Tables created using CREATE TABLE AS are initially populated with the

<p>There are many benefits to this approach, including improved
performance, reduced cost and complexity, and
improved reliability.  See technical notes
[file-format benefits|"aff_short.html"] and 
[application file-format|"appfileformat.html"] and
[faster than the filesystem|"fasterthanfs.html"] for more information.
This use case is closely related to the
<a href="#wireproto">data transfer format</a> and
<a href="#container">data container</a> use cases below.
</li>

<tcl>hd_fragment website {using SQLite for websites}</tcl>
<li><p><b>Websites</b></p>

<p>SQLite works great as the database engine for most low to
medium traffic websites (which is to say, most websites).
The amount of web traffic that SQLite can handle depends

SQL database engine in this scenario.
Database requests are serialized by the server, so concurrency is not
an issue.  Concurrency is also improved by "database sharding":
using separate database files for different subdomains.  For
example, the server might have a separate SQLite database for each
user, so that the server can handle hundreds or thousands of simultaneous
connections, but each SQLite database is only used by one connection.</p>
</li>

<tcl>hd_fragment wireproto {data transfer format}</tcl>
<li><p><b>Data transfer format</b><p>

<p>Because an SQLite database is a single compact file in a
[file format|well-defined cross-platform format], it is often used
as a container for transfering content from one system to another.
The sender gathers content into an SQLite database file, transfers
that one file to the receiver, then the receiver uses SQL to extract
the content as needed.

<p>An SQLite database facilitates data transfer between systems even
when the endpoints have different word sizes and/or byte orders.
The data can be a complex mix of large binary blobs, text, and small
numeric or boolean values.  The data format can be easily extended
by adding new tables and/or columns, without breaking legacy receivers.
The SQL query language means that receivers are not required to parse
the entire transfer all at once, but can instead query the
received content as needed.  The data format is "transparent" in the
sense that it is easily decoded for human viewing using 
a variety of universally available, open-source tools, from multiple
vendors.
</li>

<tcl>hd_fragment container {data container}</tcl>
<li><p><b>File archive and/or data container</b></p>

<p>
The [SQLite Archive] idea shows how
SQLite can be used as a substitute for ZIP archives or Tarballs.
An archive of files stored in SQLite is only very slightly larger, and
in some cases actually smaller, than the equivalent ZIP archive.

</li>

<li><p><b>High Concurrency</b></p>

<p>
SQLite supports an unlimited number of simultaneous readers, but it 
will only allow one writer at any instant in time.
For many situations, this is not a problem.  Writers queue up. Each application
does its database work quickly and moves on, and no lock lasts for more
than a few dozen milliseconds. But there are some applications that require
more concurrency, and those applications may need to seek a different
solution.
</p>
</li>

<title>Why SQLite Does Not Use Git</title>

<table_of_contents>

<h1>Introduction</h1>

<p>
SQLite does not use the