|
|
相关文章推荐
Basic module usage ¶
The basic Psycopg usage is common to all the database adapters implementing the DB API 2.0 protocol. Here is an interactive session showing some of the basic commands:
>>> import psycopg2
# Connect to an existing database
>>> conn = psycopg2.connect("dbname=test user=postgres")
# Open a cursor to perform database operations
>>> cur = conn.cursor()
# Execute a command: this creates a new table
>>> cur.execute("CREATE TABLE test (id serial PRIMARY KEY, num integer, data varchar);")
# Pass data to fill a query placeholders and let Psycopg perform
# the correct conversion (no more SQL injections!)
>>> cur.execute("INSERT INTO test (num, data) VALUES (%s, %s)",
... (100, "abc'def"))
# Query the database and obtain data as Python objects
>>> cur.execute("SELECT * FROM test;")
>>> cur.fetchone()
(1, 100, "abc'def")
# Make the changes to the database persistent
>>> conn.commit()
# Close communication with the database
>>> cur.close()
>>> conn.close()
The main entry points of Psycopg are:
The function connect() creates a new database session and
returns a new connection instance.
The class connection encapsulates a database session. It allows to:
create new cursor instances using the cursor() method to
execute database commands and queries,
terminate transactions using the methods commit() or
rollback().
The class cursor allows interaction with the database:
send commands to the database using methods such as execute()
and executemany(),
retrieve data from the database by iteration or
using methods such as fetchone(), fetchmany(),
fetchall().
Passing parameters to SQL queries¶
Psycopg converts Python variables to SQL values using their types: the Python
type determines the function used to convert the object into a string
representation suitable for PostgreSQL. Many standard Python types are
already adapted to the correct SQL representation.
Passing parameters to an SQL statement happens in functions such as
cursor.execute() by using %s placeholders in the SQL statement, and
passing a sequence of values as the second argument of the function. For
example the Python function call:
>>> cur.execute("""
... INSERT INTO some_table (an_int, a_date, a_string)
... VALUES (%s, %s, %s);
... """,
... (10, datetime.date(2005, 11, 18), "O'Reilly"))
is converted into a SQL command similar to:
INSERT INTO some_table (an_int, a_date, a_string)
VALUES (10, '2005-11-18', 'O''Reilly');
Named arguments are supported too using %(name)s placeholders in the
query and specifying the values into a mapping. Using named arguments allows
to specify the values in any order and to repeat the same value in several
places in the query:
>>> cur.execute("""
... INSERT INTO some_table (an_int, a_date, another_date, a_string)
... VALUES (%(int)s, %(date)s, %(date)s, %(str)s);
... """,
... {'int': 10, 'str': "O'Reilly", 'date': datetime.date(2005, 11, 18)})
Using characters %, (, ) in the argument names is not supported.
When parameters are used, in order to include a literal % in the query you
can use the %% string:
>>> cur.execute("SELECT (%s % 2) = 0 AS even", (10,)) # WRONG
>>> cur.execute("SELECT (%s %% 2) = 0 AS even", (10,)) # correct
While the mechanism resembles regular Python strings manipulation, there are a
few subtle differences you should care about when passing parameters to a
query.
The Python string operator % must not be used: the execute()
method accepts a tuple or dictionary of values as second parameter.
Never use % or + to merge values
into queries:
>>> cur.execute("INSERT INTO numbers VALUES (%s, %s)" % (10, 20)) # WRONG
>>> cur.execute("INSERT INTO numbers VALUES (%s, %s)", (10, 20)) # correct
For positional variables binding, the second argument must always be a
sequence, even if it contains a single variable (remember that Python
requires a comma to create a single element tuple):
>>> cur.execute("INSERT INTO foo VALUES (%s)", "bar") # WRONG
>>> cur.execute("INSERT INTO foo VALUES (%s)", ("bar")) # WRONG
>>> cur.execute("INSERT INTO foo VALUES (%s)", ("bar",)) # correct
>>> cur.execute("INSERT INTO foo VALUES (%s)", ["bar"]) # correct
The placeholder must not be quoted. Psycopg will add quotes where needed:
>>> cur.execute("INSERT INTO numbers VALUES ('%s')", (10,)) # WRONG
>>> cur.execute("INSERT INTO numbers VALUES (%s)", (10,)) # correct
The variables placeholder must always be a %s, even if a different
placeholder (such as a %d for integers or %f for floats) may look
more appropriate:
>>> cur.execute("INSERT INTO numbers VALUES (%d)", (10,)) # WRONG
>>> cur.execute("INSERT INTO numbers VALUES (%s)", (10,)) # correct
Only query values should be bound via this method: it shouldn’t be used to
merge table or field names to the query (Psycopg will try quoting the table
name as a string value, generating invalid SQL). If you need to generate
dynamically SQL queries (for instance choosing dynamically a table name)
you can use the facilities provided by the psycopg2.sql module:
>>> cur.execute("INSERT INTO %s VALUES (%s)", ('numbers', 10)) # WRONG
>>> cur.execute( # correct
... SQL("INSERT INTO {} VALUES (%s)").format(Identifier('numbers')),
... (10,))
The problem with the query parameters¶
The SQL representation of many data types is often different from their Python
string representation. The typical example is with single quotes in strings:
in SQL single quotes are used as string literal delimiters, so the ones
appearing inside the string itself must be escaped, whereas in Python single
quotes can be left unescaped if the string is delimited by double quotes.
Because of the difference, sometime subtle, between the data types
representations, a naïve approach to query strings composition, such as using
Python strings concatenation, is a recipe for terrible problems:
>>> SQL = "INSERT INTO authors (name) VALUES ('%s');" # NEVER DO THIS
>>> data = ("O'Reilly", )
>>> cur.execute(SQL % data) # THIS WILL FAIL MISERABLY
ProgrammingError: syntax error at or near "Reilly"
LINE 1: INSERT INTO authors (name) VALUES ('O'Reilly')
If the variables containing the data to send to the database come from an
untrusted source (such as a form published on a web site) an attacker could
easily craft a malformed string, either gaining access to unauthorized data or
performing destructive operations on the database. This form of attack is
called SQL injection and is known to be one of the most widespread forms of
attack to database servers. Before continuing, please print this page as a
memo and hang it onto your desk.
Psycopg can automatically convert Python objects to and from SQL
literals: using this feature your code will be more robust and
reliable. We must stress this point:
Warning
Never, never, NEVER use Python string concatenation (+) or
string parameters interpolation (%) to pass variables to a SQL query
string. Not even at gunpoint.
The correct way to pass variables in a SQL command is using the second
argument of the execute() method:
>>> SQL = "INSERT INTO authors (name) VALUES (%s);" # Note: no quotes
>>> data = ("O'Reilly", )
>>> cur.execute(SQL, data) # Note: no % operator
Values containing backslashes and LIKE¶
Unlike in Python, the backslash (\) is not used as an escape
character except in patterns used with LIKE and ILIKE where they
are needed to escape the % and _ characters.
This can lead to confusing situations:
>>> path = r'C:\Users\Bobby.Tables'
>>> cur.execute('INSERT INTO mytable(path) VALUES (%s)', (path,))
>>> cur.execute('SELECT * FROM mytable WHERE path LIKE %s', (path,))
>>> cur.fetchall()
The solution is to specify an ESCAPE character of '' (empty string)
in your LIKE query:
>>> cur.execute("SELECT * FROM mytable WHERE path LIKE %s ESCAPE ''", (path,))
Adaptation of Python values to SQL types¶
Many standard Python types are adapted into SQL and returned as Python
objects when a query is executed.
The following table shows the default mapping between Python and PostgreSQL
types:
Python
PostgreSQL
See also
float
double
smallint
integer
bigint
numeric
unicode
varchar
buffer
Buffer protocol
bytea
timetz
datetime
timestamp
timestamptz
timedelta
interval
ARRAY
tuple
namedtuple
Composite types
IN syntax
hstore
Psycopg’s Range
range
Anything™
ipaddress
objects
The mapping is fairly customizable: see Adapting new Python types to SQL syntax and
Type casting of SQL types into Python objects. You can also find a few other
specialized adapters in the psycopg2.extras module.
Constants adaptation¶
Python None and boolean values True and False are converted into the
proper SQL literals:
>>> cur.mogrify("SELECT %s, %s, %s;", (None, True, False))
'SELECT NULL, true, false;'
Numbers adaptation¶
Python numeric objects int, long, float, Decimal are
converted into a PostgreSQL numerical representation:
>>> cur.mogrify("SELECT %s, %s, %s, %s;", (10, 10L, 10.0, Decimal("10.00")))
'SELECT 10, 10, 10.0, 10.00;'
Reading from the database, integer types are converted into int, floating
point types are converted into float, numeric/decimal are
converted into Decimal.
Sometimes you may prefer to receive numeric data as float
instead, for performance reason or ease of manipulation: you can configure
an adapter to cast PostgreSQL numeric to Python float.
This of course may imply a loss of precision.
See also
Strings adaptation¶
Python str and unicode are converted into the SQL string syntax.
unicode objects (str in Python 3) are encoded in the connection
encoding before sending to the backend: trying to send a
character not supported by the encoding will result in an error. Data is
usually received as str (i.e. it is decoded on Python 3, left encoded
on Python 2). However it is possible to receive unicode on Python 2 too:
see Unicode handling.
Unicode handling¶
Psycopg can exchange Unicode data with a PostgreSQL database. Python
unicode objects are automatically encoded in the client encoding
defined on the database connection (the PostgreSQL encoding, available in
connection.encoding, is translated into a Python encoding using the
encodings mapping):
>>> print(u, type(u))
àèìòù€ <type 'unicode'>
>>> cur.execute("INSERT INTO test (num, data) VALUES (%s,%s);", (74, u))
When reading data from the database, in Python 2 the strings returned are
usually 8 bit str objects encoded in the database client encoding:
>>> print(conn.encoding)
>>> cur.execute("SELECT data FROM test WHERE num = 74")
>>> x = cur.fetchone()[0]
>>> print(x, type(x), repr(x))
àèìòù€ <type 'str'> '\xc3\xa0\xc3\xa8\xc3\xac\xc3\xb2\xc3\xb9\xe2\x82\xac'
>>> conn.set_client_encoding('LATIN9')
>>> cur.execute("SELECT data FROM test WHERE num = 74")
>>> x = cur.fetchone()[0]
>>> print(type(x), repr(x))
<type 'str'> '\xe0\xe8\xec\xf2\xf9\xa4'
In Python 3 instead the strings are automatically decoded in the connection
encoding, as the str object can represent Unicode characters.
In Python 2 you must register a typecaster in order to receive unicode objects:
>>> psycopg2.extensions.register_type(psycopg2.extensions.UNICODE, cur)
>>> cur.execute("SELECT data FROM test WHERE num = 74")
>>> x = cur.fetchone()[0]
>>> print(x, type(x), repr(x))
àèìòù€ <type 'unicode'> u'\xe0\xe8\xec\xf2\xf9\u20ac'
In the above example, the UNICODE typecaster is
registered only on the cursor. It is also possible to register typecasters on
the connection or globally: see the function
register_type() and
Type casting of SQL types into Python objects for details.
In Python 2, if you want to uniformly receive all your database input in
Unicode, you can register the related typecasters globally as soon as
Psycopg is imported:
import psycopg2.extensions
psycopg2.extensions.register_type(psycopg2.extensions.UNICODE)
psycopg2.extensions.register_type(psycopg2.extensions.UNICODEARRAY)
and forget about this story.
In some cases, on Python 3, you may want to receive bytes instead of
str, without undergoing to any decoding. This is especially the case if
the data in the database is in mixed encoding. The
BYTES caster is what you neeed:
import psycopg2.extensions
psycopg2.extensions.register_type(psycopg2.extensions.BYTES, conn)
psycopg2.extensions.register_type(psycopg2.extensions.BYTESARRAY, conn)
cur = conn.cursor()
cur.execute("select %s::text", (u"€",))
cur.fetchone()[0]
b'\xe2\x82\xac'
Binary adaptation¶
Python types representing binary objects are converted into PostgreSQL binary
string syntax, suitable for bytea fields. Such types are buffer
(only available in Python 2), memoryview, bytearray, and bytes (only in
Python 3: the name is available in Python 2 but it’s only an alias for the
type str). Any object implementing the Revised Buffer Protocol should
be usable as binary type. Received data is returned as buffer (in Python 2)
or memoryview (in Python 3).
Changed in version 2.4: only strings were supported before.
Changed in version 2.4.1: can parse the ‘hex’ format from 9.0 servers without relying on the
version of the client library.
In Python 2, if you have binary data in a str object, you can pass them
to a bytea field using the psycopg2.Binary wrapper:
mypic = open('picture.png', 'rb').read()
curs.execute("insert into blobs (file) values (%s)",
(psycopg2.Binary(mypic),))
Warning
Since version 9.0 PostgreSQL uses by default a new “hex” format to
emit bytea fields. Starting from Psycopg 2.4.1 the format is
correctly supported. If you use a previous version you will need some
extra care when receiving bytea from PostgreSQL: you must have at least
libpq 9.0 installed on the client or alternatively you can set the
bytea_output configuration parameter to escape, either in the
server configuration file or in the client session (using a query such as
SET bytea_output TO escape;) before receiving binary data.
Date/Time objects adaptation¶
Python builtin datetime, date,
time, timedelta are converted into PostgreSQL’s
timestamp[tz], date, time[tz], interval data types.
Time zones are supported too.
>>> dt = datetime.datetime.now()
datetime.datetime(2010, 2, 8, 1, 40, 27, 425337)
>>> cur.mogrify("SELECT %s, %s, %s;", (dt, dt.date(), dt.time()))
"SELECT '2010-02-08T01:40:27.425337', '2010-02-08', '01:40:27.425337';"
>>> cur.mogrify("SELECT %s;", (dt - datetime.datetime(2010,1,1),))
"SELECT '38 days 6027.425337 seconds';"
See also
Time zones handling¶
The PostgreSQL type timestamp with time zone (a.k.a.
timestamptz) is converted into Python datetime objects.
>>> cur.execute("SET TIME ZONE 'Europe/Rome'") # UTC + 1 hour
>>> cur.execute("SELECT '2010-01-01 10:30:45'::timestamptz")
>>> cur.fetchone()[0]
datetime.datetime(2010, 1, 1, 10, 30, 45,
tzinfo=datetime.timezone(datetime.timedelta(seconds=3600)))
Before Python 3.7, the datetime module only supported timezones with an
integer number of minutes. A few historical time zones had seconds in the
UTC offset: these time zones will have the offset rounded to the nearest
minute, with an error of up to 30 seconds, on Python versions before 3.7.
>>> cur.execute("SET TIME ZONE 'Asia/Calcutta'") # offset was +5:21:10
>>> cur.execute("SELECT '1900-01-01 10:30:45'::timestamptz")
>>> cur.fetchone()[0].tzinfo
# On Python 3.6: 5h, 21m
datetime.timezone(datetime.timedelta(0, 19260))
# On Python 3.7 and following: 5h, 21m, 10s
datetime.timezone(datetime.timedelta(seconds=19270))
Changed in version 2.2.2: timezones with seconds are supported (with rounding). Previously such
timezones raised an error.
Changed in version 2.9: timezones with seconds are supported without rounding.
Changed in version 2.9: use datetime.timezone as default tzinfo object instead of
FixedOffsetTimezone.
Infinite dates handling¶
PostgreSQL can store the representation of an “infinite” date, timestamp, or
interval. Infinite dates are not available to Python, so these objects are
mapped to date.max, datetime.max, interval.max. Unfortunately the
mapping cannot be bidirectional so these dates will be stored back into the
database with their values, such as 9999-12-31.
It is possible to create an alternative adapter for dates and other objects
to map date.max to infinity, for instance:
class InfDateAdapter:
def __init__(self, wrapped):
self.wrapped = wrapped
def getquoted(self):
if self.wrapped == datetime.date.max:
return b"'infinity'::date"
elif self.wrapped == datetime.date.min:
return b"'-infinity'::date"
else:
return psycopg2.extensions.DateFromPy(self.wrapped).getquoted()
psycopg2.extensions.register_adapter(datetime.date, InfDateAdapter)
Of course it will not be possible to write the value of date.max in the
database anymore: infinity will be stored instead.
Time handling¶
The PostgreSQL time and Python time types are not
fully bidirectional.
Within PostgreSQL, the time type’s maximum value of 24:00:00 is
treated as 24-hours later than the minimum value of 00:00:00.
>>> cur.execute("SELECT '24:00:00'::time - '00:00:00'::time")
>>> cur.fetchone()[0]
datetime.timedelta(days=1)
However, Python’s time only supports times until 23:59:59.
Retrieving a value of 24:00:00 results in a time of 00:00:00.
>>> cur.execute("SELECT '24:00:00'::time, '00:00:00'::time")
>>> cur.fetchone()
(datetime.time(0, 0), datetime.time(0, 0))
Lists adaptation¶
Python lists are converted into PostgreSQL ARRAYs:
>>> cur.mogrify("SELECT %s;", ([10, 20, 30], ))
'SELECT ARRAY[10,20,30];'
You can use a Python list as the argument of the IN operator using
the PostgreSQL ANY operator.
ids = [10, 20, 30]
cur.execute("SELECT * FROM data WHERE id = ANY(%s);", (ids,))
Furthermore ANY can also work with empty lists, whereas IN ()
is a SQL syntax error.
Reading back from PostgreSQL, arrays are converted to lists of Python
objects as expected, but only if the items are of a known type.
Arrays of unknown types are returned as represented by the database (e.g.
{a,b,c}). If you want to convert the items into Python objects you can
easily create a typecaster for array of unknown types.
Tuples adaptation¶
Python tuples are converted into a syntax suitable for the SQL IN
operator and to represent a composite type:
>>> cur.mogrify("SELECT %s IN %s;", (10, (10, 20, 30)))
'SELECT 10 IN (10, 20, 30);'
SQL doesn’t allow an empty list in the IN operator, so your code
should guard against empty tuples. Alternatively you can use a
Python list.
If you want PostgreSQL composite types to be converted into a Python
tuple/namedtuple you can use the register_composite()
function.
New in version 2.0.6: the tuple IN adaptation.
Changed in version 2.0.14: the tuple IN adapter is always active. In previous releases it
was necessary to import the extensions module to have it
registered.
Changed in version 2.3: namedtuple instances are adapted like regular tuples and
can thus be used to represent composite types.
Transactions control¶
In Psycopg transactions are handled by the connection class. By
default, the first time a command is sent to the database (using one of the
cursors created by the connection), a new transaction is created.
The following database commands will be executed in the context of the same
transaction – not only the commands issued by the first cursor, but the ones
issued by all the cursors created by the same connection. Should any command
fail, the transaction will be aborted and no further command will be executed
until a call to the rollback() method.
The connection is responsible for terminating its transaction, calling either
the commit() or rollback() method. Committed
changes are immediately made persistent in the database. If the connection
is closed (using the close() method) or destroyed (using del
or by letting it fall out of scope) while a transaction is in progress, the
server will discard the transaction. However doing so is not advisable:
middleware such as PgBouncer may see the connection closed uncleanly and
dispose of it.
It is possible to set the connection in autocommit mode: this way all the
commands executed will be immediately committed and no rollback is possible. A
few commands (e.g. CREATE DATABASE, VACUUM, CALL on
stored procedures using transaction control…) require to be run
outside any transaction: in order to be able to run these commands from
Psycopg, the connection must be in autocommit mode: you can use the
autocommit property.
Warning
By default even a simple SELECT will start a transaction: in
long-running programs, if no further action is taken, the session will
remain “idle in transaction”, an undesirable condition for several
reasons (locks are held by the session, tables bloat…). For long lived
scripts, either make sure to terminate a transaction as soon as possible or
use an autocommit connection.
A few other transaction properties can be set session-wide by the
connection: for instance it is possible to have read-only transactions or
change the isolation level. See the set_session() method for all
the details.
with statement¶
Starting from version 2.5, psycopg2’s connections and cursors are context
managers and can be used with the with statement:
with psycopg2.connect(DSN) as conn:
with conn.cursor() as curs:
curs.execute(SQL)
When a connection exits the with block, if no exception has been raised by
the block, the transaction is committed. In case of exception the transaction
is rolled back.
When a cursor exits the with block it is closed, releasing any resource
eventually associated with it. The state of the transaction is not affected.
A connection can be used in more than one with statement
and each with block is effectively wrapped in a separate transaction:
conn = psycopg2.connect(DSN)
with conn:
with conn.cursor() as curs:
curs.execute(SQL1)
with conn:
with conn.cursor() as curs:
curs.execute(SQL2)
conn.close()
Warning
Unlike file objects or other resources, exiting the connection’s
with block doesn’t close the connection, but only the transaction
associated to it. If you want to make sure the connection is closed after
a certain point, you should still use a try-catch block:
conn = psycopg2.connect(DSN)
try:
# connection usage
finally:
conn.close()
Server side cursors¶
When a database query is executed, the Psycopg cursor usually fetches
all the records returned by the backend, transferring them to the client
process. If the query returns a huge amount of data, a proportionally large
amount of memory will be allocated by the client.
If the dataset is too large to be practically handled on the client side, it is
possible to create a server side cursor. Using this kind of cursor it is
possible to transfer to the client only a controlled amount of data, so that a
large dataset can be examined without keeping it entirely in memory.
Server side cursor are created in PostgreSQL using the DECLARE command and
subsequently handled using MOVE, FETCH and CLOSE commands.
Psycopg wraps the database server side cursor in named cursors. A named
cursor is created using the cursor() method specifying the
name parameter. Such cursor will behave mostly like a regular cursor,
allowing the user to move in the dataset using the scroll()
method and to read the data using fetchone() and
fetchmany() methods. Normally you can only scroll forward in a
cursor: if you need to scroll backwards you should declare your cursor
scrollable.
Named cursors are also iterable like regular cursors.
Note however that before Psycopg 2.4 iteration was performed fetching one
record at time from the backend, resulting in a large overhead. The attribute
itersize now controls how many records are fetched at time
during the iteration: the default value of 2000 allows to fetch about 100KB
per roundtrip assuming records of 10-20 columns of mixed number and strings;
you may decrease this value if you are dealing with huge records.
Named cursors are usually created WITHOUT HOLD, meaning they live only
as long as the current transaction. Trying to fetch from a named cursor after
a commit() or to create a named cursor when the connection
is in autocommit mode will result in an exception.
It is possible to create a WITH HOLD cursor by specifying a True
value for the withhold parameter to cursor() or by setting the
withhold attribute to True before calling execute() on
the cursor. It is extremely important to always close() such cursors,
otherwise they will continue to hold server-side resources until the connection
will be eventually closed. Also note that while WITH HOLD cursors
lifetime extends well after commit(), calling
rollback() will automatically close the cursor.
It is also possible to use a named cursor to consume a cursor created
in some other way than using the DECLARE executed by
execute(). For example, you may have a PL/pgSQL function
returning a cursor:
CREATE FUNCTION reffunc(refcursor) RETURNS refcursor AS $$
BEGIN
OPEN $1 FOR SELECT col FROM test;
RETURN $1;
END;
$$ LANGUAGE plpgsql;
You can read the cursor content by calling the function with a regular,
non-named, Psycopg cursor:
cur1 = conn.cursor()
cur1.callproc('reffunc', ['curname'])
and then use a named cursor in the same transaction to “steal the cursor”:
cur2 = conn.cursor('curname')
for record in cur2: # or cur2.fetchone, fetchmany...
# do something with record
Thread and process safety¶
The Psycopg module and the connection objects are thread-safe: many
threads can access the same database either using separate sessions and
creating a connection per thread or using the same
connection and creating separate cursors. In DB API 2.0 parlance, Psycopg is
level 2 thread safe.
The difference between the above two approaches is that, using different
connections, the commands will be executed in different sessions and will be
served by different server processes. On the other hand, using many cursors on
the same connection, all the commands will be executed in the same session
(and in the same transaction if the connection is not in autocommit mode), but they will be serialized.
The above observations are only valid for regular threads: they don’t apply to
forked processes nor to green threads. libpq connections shouldn’t be used by a
forked processes, so when using a module such as multiprocessing or a
forking web deploy method such as FastCGI make sure to create the connections
after the fork.
Connections shouldn’t be shared either by different green threads: see
Support for coroutine libraries for further details.
Using COPY TO and COPY FROM¶
Psycopg cursor objects provide an interface to the efficient
PostgreSQL COPY command to move data from files to tables and back.
Currently no adaptation is provided between Python and PostgreSQL types on
COPY: the file can be any Python file-like object but its format must be in
the format accepted by PostgreSQL COPY command (data format, escaped
characters, etc).
The methods exposed are:
copy_from()Reads data from a file-like object appending them to a database table
(COPY table FROM file syntax). The source file must provide both
read() and readline() method.
copy_to()Writes the content of a table to a file-like object (COPY table TO
file syntax). The target file must have a write() method.
copy_expert()Allows to handle more specific cases and to use all the COPY
features available in PostgreSQL.
Please refer to the documentation of the single methods for details and
examples.
Access to PostgreSQL large objects¶
PostgreSQL offers support for large objects, which provide stream-style
access to user data that is stored in a special large-object structure. They
are useful with data values too large to be manipulated conveniently as a
whole.
Psycopg allows access to the large object using the
lobject class. Objects are generated using the
connection.lobject() factory method. Data can be retrieved either as bytes
or as Unicode strings.
Psycopg large object support efficient import/export with file system files
using the lo_import() and lo_export() libpq functions.
Changed in version 2.6: added support for large objects greater than 2GB. Note that the support is
enabled only if all the following conditions are verified:
the Python build is 64 bits;
the extension was built against at least libpq 9.3;
the server version is at least PostgreSQL 9.3
(server_version must be >= 90300).
If Psycopg was built with 64 bits large objects support (i.e. the first
two conditions above are verified), the psycopg2.__version__ constant
will contain the lo64 flag. If any of the contition is not met
several lobject methods will fail if the arguments exceed 2GB.
Two-Phase Commit protocol support¶
New in version 2.3.
Psycopg exposes the two-phase commit features available since PostgreSQL 8.1
implementing the two-phase commit extensions proposed by the DB API 2.0.
The DB API 2.0 model of two-phase commit is inspired by the XA specification,
according to which transaction IDs are formed from three components:
a format ID (non-negative 32 bit integer)
a global transaction ID (string not longer than 64 bytes)
a branch qualifier (string not longer than 64 bytes)
For a particular global transaction, the first two components will be the same
for all the resources. Every resource will be assigned a different branch
qualifier.
According to the DB API 2.0 specification, a transaction ID is created using the
connection.xid() method. Once you have a transaction id, a distributed
transaction can be started with connection.tpc_begin(), prepared using
tpc_prepare() and completed using tpc_commit() or
tpc_rollback(). Transaction IDs can also be retrieved from the
database using tpc_recover() and completed using the above
tpc_commit() and tpc_rollback().
PostgreSQL doesn’t follow the XA standard though, and the ID for a PostgreSQL
prepared transaction can be any string up to 200 characters long.
Psycopg’s Xid objects can represent both XA-style
transactions IDs (such as the ones created by the xid() method) and
PostgreSQL transaction IDs identified by an unparsed string.
The format in which the Xids are converted into strings passed to the
database is the same employed by the PostgreSQL JDBC driver: this should
allow interoperation between tools written in Python and in Java. For example
a recovery tool written in Python would be able to recognize the components of
transactions produced by a Java program.
For further details see the documentation for the above methods.
Passing parameters to SQL queries
推荐文章