Documentation Source Text

RecursiveBubbleDiagram begin-stmt
RecursiveBubbleDiagram commit-stmt
RecursiveBubbleDiagram rollback-stmt
</tcl>

<p>
^No reads or writes occur except within a transaction.
^Any command that accesses the database (basically, any SQL command,
except a few [PRAGMA] statements)
will automatically start a transaction if
one is not already in effect.  ^Automatically started transactions
are committed when the last SQL statement finishes.
</p>

<p>
^Transactions can be started manually using the BEGIN
command.  ^(Such transactions usually persist until the next
COMMIT or ROLLBACK command.  But a transaction will also 
ROLLBACK if the database is closed or if an error occurs

in the syntax diagram above is only applicable to [SAVEPOINT]
transactions.  ^An attempt to invoke the BEGIN command within
a transaction will fail with an error, regardless of whether
the transaction was started by [SAVEPOINT] or a prior BEGIN.
^The COMMIT command and the ROLLBACK command without the TO clause
work the same on [SAVEPOINT] transactions as they do with transactions
started by BEGIN.</p>

<h3>Read transactions versus write transactions</h3>

<p>SQLite current supports multiple simultaneous read transactions
coming from separate database connections, possibly in separate
threads or processes, but only one simultaneous write transaction.
<p>

<p>A read transaction is used for reading only.  A write transaction
allows both reading and writing.  A read transaction is started
by a SELECT statement, and a write transaction is started by 
statements like CREATE, DELETE, DROP, INSERT, or UPDATE (collectively
"write statements").  If a write statement occurs while
a read transaction is active, then the read transaction is upgraded
to write transaction if possible.  If some other database connection
has already modified the database or is already in the process of
modifying the database, then upgrading to a write transaction is
not possible and the write statement will fail with [SQLITE_BUSY].
</p>

<p>
While a read transaction is active, any changes to the database that
are implemented by separate database connections will not be seen
by the database connection that started the read transaction.  If database
connection X is holding a read transaction, it is possible that some
other database connection Y might change the content of the database
while X's transaction is still open, however X will not be able to see 
those changes until after the transaction ends.  While its read
transaction is active, X will continue to see an historic snapshot
the database prior to the changes implemented by Y.
</p>


<tcl>hd_fragment immediate {BEGIN IMMEDIATE} {BEGIN EXCLUSIVE}</tcl>
<h3>DEFERRED, IMMEDIATE, and EXCLUSIVE transactions</h3>

<p>
^Transactions can be DEFERRED, IMMEDIATE, or EXCLUSIVE.
^The default transaction behavior is DEFERRED.
</p>

<p>
^DEFERRED means that the transaction does not actually
start until the database is first accessed.  ^Internally,
the BEGIN DEFERRRED statement merely sets a flag on the database 
connection that turns off the automatic commit that would normally
occur when the last statement finishes.  This causes the transaction
that is automatically started to persist until an explicit
COMMIT or ROLLBACK or until a rollback is provoked by an error
or an ON CONFLICT ROLLBACK clause.  If the first statement after
BEGIN DEFERRED is a SELECT, then a read transaction is started.
Subsequent write statements will upgrade the transaction to a
write transaction if possible, or return SQLITE_BUSY.  If the
first statement after BEGIN DEFERRED is a write statement, then
a write transaction is started.
</p>

<p>
^IMMEDIATE cause the database connection to start a new write
immediately, without waiting for a writes statement.  The 
BEGIN IMMEDIATE might fail with [SQLITE_BUSY] if another write
transaction is already active on another database connection.
</p>

<p>












^EXCLUSIVE is similar to IMMEDIATE in that a write transaction


is started immediately.  EXCLUSIVE and IMMEDIATE are the same



in [WAL mode], but in other journaling modes, EXCLUSIVE prevents
other database connections from reading the database while the

transaction is underway.

</p>

<h3>Implicit versus explicit transactions</h3>

<p>
An implicit transaction (a transaction that is started automatically,
not a transaction started by BEGIN) is committed automatically when
the last active statement finishes.  A statement finishes when its
last cursor closes, which is guaranteed to happen when the
prepared statement is [sqlite3_reset() | reset] or
[sqlite3_finalize() | finalized].  Some statements might "finish"
for the purpose of transaction control prior to being reset or finalized,
but there is no guarantee of this.  The only way to ensure that a
statement as "finished" is to invoke [sqlite3_reset()] or
[sqlite3_finalize()] on that statement.  An open [sqlite3_blob] used for
incremental BLOB I/O also counts as an unfinished statement.
The [sqlite3_blob] finishes when it is [sqlite3_blob_close() | closed].
</p>

<p>
^The explicit COMMIT command runs immediately, even if there are
pending [SELECT] statements.  ^However, if there are pending
write operations, the COMMIT command
will fail with an error code [SQLITE_BUSY].
</p>

<p>
^An attempt to execute COMMIT might also result in an [SQLITE_BUSY] return code
if an another thread or process has an open read connection.
^When COMMIT fails in this
way, the transaction remains active and the COMMIT can be retried later
after the reader has had a chance to clear.
</p>

<p>
In very old versions of SQLite (before version 3.7.11 - [dateof:3.7.11])
the ROLLBACK will fail with an error code