Server side cursor psycopg2 If you have an extremely large result set to retrieve from your database, or you would like to iterate through a tables records without Psycopg allows the use of server-side cursors using the classes ServerCursor and AsyncServerCursor. connect('user=postgres') with conn. The cursor refrains from reading the next 1000 unless it scrolls past There are three essential methods for plucking out data from a Psycopg2 cursor: fetchone; fetchmany; To show the real power of a server side cursor to fetch a given number of rows as a batch Starting from version 2. db. This will allow you to perform the query without using paging (as LIMIT/OFFSET implements), and will simplify your code. fetchone() print result['count'] Because you used . cursor('name_of_the_new_server_side_cursor') cursor. Cursors are created by the connection. iterator() querysets. Python script does not try to reconnect to sql db after implementing try / exception. cursor(name='cursor_name') for large query results to avoid memory Django's cursor class is just a wrapper around the underlying DB's cursor, so the effect of leaving the cursor open is basically tied to the underlying DB driver. extra connection: Developers have to remember to use separate Client-side cursors Client-side-binding cursors Server-side cursors “Stealing” an existing cursor Psycopg can manage kinds of “cursors” which differ in where the state of a query being processed is stored: Client-side cursors and Server-side cursors. . (Given that this is triggered deep within Django, I am not sure I have the luxury of making changes there). Follow answered Feb 11, 2020 at 17:58. iterator(): item. Server-side cursors can usually scroll backwards only if declared scrollable . mogrify is just a manual invocation of exactly the same logic that psycopg2 uses when it interpolates parameters into the SQL string its self, before it sends it to there are limitations not so much in the wire protocol, but in the server itself. answered Nov 29, 2018 at 18:05. Pretty sure cursor. psycopg2 uses so-called scrollable cursors (PostgreSQL: Documentation: 16: 43. Now, the explanation for why it isn't freed, and why that isn't a memory leak in the formally correct use of that term. extras. import itertools import pandas as pd import psycopg2 import pyarrow as pa import pyarrow. query = Server-side cursors can usually scroll backwards only if declared scrollable. 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. backends. It works with normal cursors, but not with server side: import psycopg2 conn = psycopg2. itersize = 20000. RealDictCursor) The cursor seems to work in that it can be I'm profiling code to see the difference between bulk load from a Postgres DB source or stream from the source. cur = conn. 3 and psycopg2 2. python: How to use server side cursors with psycopg2Thanks for taking the time to learn more. execute(""" SELECT * FROM table LIMIT 10000000 """) while True: rows = cursor. This turns the cursor from a lightweight Postgres query stalls when selecting explicit columns with an order by using a server side cursor with psycopg2. Installation and Basic Usage. Each thread should have its own database connection. user1123335 user1123335. As it turns out, I believe the problem is more fundamental. However, by default they're forward-only, unlike psycopg2's in-memory client-side cursors, and they consume database resources until they are released. Ask Question Asked 9 years, 7 months ago. engine. connection. It features client-side and server-side cursors, asynchronous communication and notifications, "COPY TO/COPY FROM" support. Threading inside a cursor in psycopg2. Thanks for the feedback. Psycopg2 has a nice interface for working with server side cursors. 5, psycopg2’s connections and cursors are context managers and can be used with the with statement: Server side cursor are created in PostgreSQL using the DECLARE command and subsequently handled using MOVE, FETCH and CLOSE commands. A named cursor is created using the cursor() method specifying the name parameter. A server side cursor, by contrast, indicates that result rows remain pending within the How to use server side cursors with psycopg2. A client side cursor here means that the database driver fully fetches all rows from a result set into memory before returning from a statement execution. 9: previosly the default factory was psycopg2. Drivers such as those of PostgreSQL and MySQL/MariaDB generally use client side cursors by default. Try: with connection: cursor = connection. nextset Python psycopg2 cursor. if the same you should have the same timings. raw_connection() method that returns (a proxy to) the raw DBAPI connection, and calling I faced the same issue when I messed with getting it possible to use unmanaged (with managed = False in model's Meta) models in Django unittests. The best option is probably to catch both exceptions in your code:: try: cur. 5. The "server side" script I'm running is: I want to to use psycopg to create a server-side cursor to my postgres DB so that I can read a very large table. So while using fetchmany() instead of fetchall() may save some memory in terms of Python objects creation, using a server-side Server-side cursors throw an exception when close() is called before execute(). Cursors), i. psycopg2 process cursor results with muliple threads or processes. 2. When you run a query, you will get the whole response back from the server. This attribute will be None for operations that do not return rows or if the cursor has not had an operation invoked via the . How to get psycopg2's description from PostgreSQL server side cursor. cursor(name='name_of_cursor') as cursor: cursor. Cursors subclasses# In psycopg2, a few cursor subclasses allowed to return data in different form than tuples. 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. Server-side cursors can usually scroll backwards only if declared `~cursor How to use server side cursors with psycopg2. So if several threads were to share a database connection, they'd have to coordinate carefully to make sure that Also, note that this is with a server-side cursor. 0 "Memory error" when using pd. It offers advanced Way 2: Use `fetchmany` with server-side cursor. def insertLocalDB(): # Open a cursor to perform database operations cur . Server-side binding# Psycopg 3 sends the query and the parameters to the server separately, instead of merging them on the client side. To install psycopg2, use the following pip command: pip install I've left the attempted branch open as named_cursor_oob, the last commit implementation is dvarrazzo/psycopg@882eced. cursors that have to be created explicitly with DECLARE CURSOR and that can go forward and backward. # HERE IS THE IMPORTANT PART, by specifying a name for the cursor # psycopg2 creates a server-side cursor, which prevents all of the # records from being downloaded at Also keep in mind the importance of named cursors, which is where psycopg cursors and Postgres cursors intertwine. Engine'> that is associated with the session. What is correct way to use psycopg2 cursors in threads? Hot Network Questions Why does it take so long to stop the rotor of a helicopter after landing? psycopg2 doesn't use server-side prepared statements and bind parameters at all. Engine objects have a . My data volume is pretty stable, and this has been working well for some time. Get lazy but reusable cursor with Psycopg2. Moving out-of-bound in a server-side cursor doesn’t result in an exception, if the backend doesn’t raise any (Postgres doesn’t tell us in a reliable way if we went out of bound cursor = conn. I found out that giving a name to my cursor will create a server-side cursor that will only load the number of rows I will ask it to, using 'fetchmany' but it has become significantly slower to perform a query. There might be a few workarounds using psycopg3: Use a server-side cursor. They are usually created by passing the name parameter to the cursor() My understanding is that the query is executed, but 1000 of the records are read by the server side cursor. In such querying pattern, after a cursor sends a query to the server (usually calling execute()), the server replies transferring to the client the whole set of results requested, which is Server-side cursors can usually scroll backwards only if declared scrollable. Improve this answer. It is a powerful and flexible connector, which allows Python applications to execute SQL commands and handle data seamlessly. Instead, use the much more efficient cursor. The design I'm testing for stream is a server side cursor with an itersize of 20,000. According to psycopg2's (psycopg2 is DB driver Django uses for PostgreSQL DB's) FAQ, their cursors are lightweight, but will cache the data being returned from queries you made using the cursor object, which could potentially Server-side cursors can usually scroll backwards only if declared scrollable. I don't see that happening using psycopg2 and copy_expert. value if random_reason_to_break: break Setup: In your own project create the package hierarchy myproject. e. In this video, we will learn the difference between a server-side cursor and a client-side cursor. Session objects have a . connect('my connection string here') cursor = connection. The other answers here are; unfortunately, the answer and here's why. 1psycopg vs psycopg-binary The psycopg2-binarypackage is meant for beginners to start playing with Python and PostgreSQL without the need to meet the build requirements. . Psycopg wraps the Fetch Records using a Server-Side Cursor. con=psycopg2. Other cursor classes can be created Server-side cursors don't require lots of memory on the client (or server) and they can deliver the first results to the application before the whole result set has been transferred. I installed psycopg3 with pip install "psycopg[binary]" on my standard Ubuntu 20. wrap into transaction: This adds overhead of transaction and can decrease the query execution throughput on high traffic sites which uses lot of . The query basically says “fetch items from one table, that don’t appear in another table”. Server Side Cursors for Django's psycopg2 Backend. e. As per Psycopg2's server-side-cursor documentation,. Note 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(). If you are not using a dict(-like) row cursor, rows are tuples and the count value is the The method can be used both for client-side cursors and server-side cursors . The design for bulk is psycopg2 with a client side cursor. They are implemented by the Cursor and I have a stored procedure in PostgreSQL that returns a refcursor (its name can be passed as an argument): -- Example stored procedure. forward-only cursors, and that’s what you need. 2 Score: with server_side_cursors(qs, itersize=100): for item in qs. psycopg2 supports server-side cursors, that is, a cursor that is managed on the database server rather than in the client. 6 The cursor link you show refers to the Python DB API cursor not the Postgres one. When query takes more than 180 seconds the script execution hangs up for a long time. The full result set is not transferred all at once to the client, rather it is fed to it as required via the cursor interface. 6. connect() cursor_id = uuid. cursor( cursor_id, cursor_factory=cursor_factory, ) as I am creating server-side cursors at several places during the course of a long ETL process. cursor('cursor_unique_name', cursor_factory=psycopg2. with psycopg2. cursor = conn. ) I figured out I need to use server side cursor, since I can not fetch all data into memory. You might have better luck with No combinations of scrollable/withhold triggered the issue on psycopg2 nor fixed on psycopg3. fetchall() 1. In this case there is only 1 variable. hex with self. Server-Side Cursors: Use connection. In this video I'll go through your question, provide various an Using ClientCursor , Psycopg 3 behaviour will be more similar to psycopg2 (which only implements client-side binding) and could be useful to port Psycopg Using a server-side cursor it is possible to process datasets larger than what would fit in the client’s memory. fetchmany method. In psycopg2 asynchronous mode, a Psycopg Connection will rely on the caller to poll the socket file descriptor, checking if it is ready to accept data or if a query result has been transferred and is ready to be How to use server side cursors with psycopg2. bind attribute which returns the <class 'sqlalchemy. execute(query) for row in cursor: # process row An answer four years later, but it is possible to have more than one cursor open from the same connection. scroll(1000 * 1000) except (ProgrammingError, IndexError), exc: deal_with_it(exc) The method can be used both for client-side cursors and :ref:`server-side cursors <server-side-cursors>`. They are implemented by the Cursor and AsyncCursor classes. cursor() Server side (named) cursors can be used only for SELECT or VALUES queries. cursor(id, cursor_factory=psycopg2. Hot Network Questions How can Rupert Murdoch be having a problem changing the beneficiaries of his trust? Manhwa about a man who, right as he is about to die, goes back in time to the day before the zombie apocalypse Convert pipe delimited column data to HTML table format for email now examine the timings from the server side and note: is the sql reported on the server the same in each case . The solution was to make the models managed when unittests are executed. 1. cursor(name='name_of_cursor') as cursor: query = "SELECT * FROM tbl FOR UPDATE" cursor. Client-side cursors Client-side cursors are what Psycopg uses in its normal querying process. parquet as pq def get_schema_and_batches(query, chunk_size): def _batches I have a problem with executing long time queries using psycopg2 in Python. connect(database='XXXX',user='XXXX',password='XXXX',host='localhost') Psycopg wraps the database server side cursor in named cursors. Changed in version 2. base. In Psycopg 3 the same can be achieved by setting a row factory: Here is a way that uses psycopg2, server side cursors, and Pandas, to batch/chunk PostgreSQL query results and write them to a parquet file without it all being in memory at once. See the psycopg2 documentation. connect(conn_string) ### HERE IS THE IMPORTANT PART, by specifying a name for the cursor ### psycopg2 creates a server-side cursor, which prevents all of the ### records from being downloaded at once from the server. We can achieve the same result as itersize property using fetchmany with a server-side cursor to reduce the no of requests from client to Psycopg allows the use of server-side cursors using the classes ServerCursor and AsyncServerCursor. They are usually created by passing the name parameter to the cursor() Allows Python code to execute PostgreSQL command in a database session. cursor(name='cursor_name results is itself a row object, in your case (judging by the claimed print output), a dictionary (you probably configured a dict-like cursor subclass); simply access the count key:. read_sql_query method. This has slightly higher overhead on the server, but will keep memory completely flat on the client (unless you are trying to hold all the records. py import psycopg2 conn = psycopg2. cursor('my_cursor') as stmt: stmt. I am using psycopg2 and pandas to extract data from Postgres. The fetch* methods get results from the cursor on the client side. The Cursor and AsyncCursor classes are the main objects to send commands to a PostgreSQL database session. The first is to be able to represent server-side cursors for situations where the result set is larger than memory, and can't be retrieved from the DB all at once; in this case the cursor serves as the client-side interface for interacting with the server-side cursor. The test table has ~600,000 rows. I see that a server-side cursor can be used with psycopg2, but I don't see a way to connect to my Netezza database using psycopg2 or a way to change the pyodbc connection I create to use a server-side cursor. Is there a proper way to handle cursors returned from a I am using psycopg2 to query from my Postgres server, this is the code that query:. If name is specified, the returned cursor will be a server side cursor (also known as named cursor). It actually does all queries via string interpolation, but it respects quoting rules carefully and does so in a secure manner. 4. fetchone() only one row is returned, not a list of rows. execute*() method yet. Viewed 204 times 0 I have a simple query that joins two (reasonably large) tables and iterates over the results with a server side cursor: conn = psycopg2. Performance issue with psycopg2 named cursor in python. There is no way to get the description or even rowcount back from a server-side cursor without first invoking a fetch. You can change the sql statements (as in alecxe answers) but there is also pure python approach using the feature provided by psycopg2: The principle is to use named cursor in Psycopg2 and give it a good itersize to load many rows at once It offers advanced features like connection pooling, server-side cursors, and thread safety, making it suitable for both simple and complex database tasks. By default a server-side cursor's results are unavailable at the end of a transaction but Django wants them to be available to subsequent transactions because otherwise its default auto-commit mode would look silly, so it adds WITH HOLD. execute (or equivalent) is called. cursor(cursor_factory=psycopg2. fetchmany(5000) if not rows: break for row in rows: # do something with row pass The reason psycopg2 has cursors at all is twofold. Both the versions have client side and server side cursors with some different behaviors but psycopg3 introduced Async cursor too. Share. cursor. A PostgreSQL connection can handle only one statement at a given time (unless you are using a server side cursor, but even then the connection can handle only one FETCH at the same time). I want to automatically close the db connection once all rows are fetched from a server-side cursor. Moving out-of-bound in a server-side cursor doesn’t result in an exception, if the backend doesn’t raise Server side cursor are created in PostgreSQL using the DECLARE command and subsequently handled using MOVE, FETCH and CLOSE commands. Follow edited Jan 6, 2023 at 1:49. Postgres SQL - CURSOR WITHOUT HOLD FOR CREATE TYPE. The Node. psycopg2 using too much memory. Cursor classes#. Instead of doing that manually at every place the cursor is created, I want to define a custom psycopg2 cursor class that closes the connection on User a server-side cursor. Excessive memory usage while getting data from a Postgres database. Simply by giving the name attribute a value in the constructor call, you will get a server-side cursor automatically which can then be iterated over just as any Python collection would, and which performs chunked fetches. tz module. They are normally created by the connection’s cursor() method. "postgresql_psycopg2_"-prefix or not, if we let Select have a "_use_server_side_cursor"-attribute, whether to use it can be determined during compilation of the statement. and I also figured out I need to use two connections so when I commit I dont loose the cursor that I made. And I think the binary version of psycopg3 is not ABI compatible with that system. You seem to have explored this solution already in psycopg2 and I wouldn't expect substantial differences. Generally you'd use imap_unordered to iterate over a collection of single items (and use a higher chunksize than the default 1), but I think we can just as well use the batches here # Open a cursor to perform database operations cur=conn. (It may be that the library was updated to fix the problem above. Return a new cursor object using the connection. 4. If you want pagination could must either construct the appropriate queries on the client side or use a server-side cursor. js pg package allows me to do the following where providing a name (insert-values) prepares the query server-side: There are tens of millions of matched rows so I am currently running out of memory before my query can complete. DictCursor) # tell postgres to use more work memory work_mem = 2048 # by passing a tuple as the 2nd argument to the execution function our # %s string variable will get replaced with the order of variables in # the list. If the dataset is too large to be practically handled on the client side, it is possible to create a server side cursor. with conn. They implement Postgres cursors: query psycopg2 is a widely used Python library designed to facilitate communication with PostgreSQL databases, offering a robust and efficient way to perform various database operations. fetchall() returns empty list but cursor. A few implementations are available in the psycopg2. However, not using a server side cursor (passing name to cursor function) in psycopg3 fixed the issue. uuid4(). cursor() # Execute a query cur. cursor() method: they are bound to the connection for the entire lifetime and Psycopg2 is a popular Python adapter for PostgreSQL, enabling seamless interaction between Python applications and PostgreSQL databases. Many Python types are supported out-of-the-box and adapted to matching PostgreSQL data types; adaptation can be extended and customized thanks to a flexible objects adaptation system. If you want to process the data in buckets, use fetchmany() in a loop, e. is one driver doing a lot of casting/converting between character sets or implicit converting of other types such as dates or The concept of server side cursors are not really Postgres-specific, though, and can possibly be reused by other dialects, when they get support for it. copy_expert(f"COPY (SELECT * FROM ONLY {table_name}) TO STDOUT WITH CSV HEADER", csv_file) is just going to stream everything directly to the file. psycopg2: RE-USE of a cursor for a RE-RUN of a SELECT query after doing an UPDATE in between the SELECT queries. hex connection = psycopg2. CREATE OR REPLACE FUNCTION example_stored_procedure(ref I'm using server-side cursor in PostgreSQL with psycopg2, based on this well-explained answer. However for small queries they are less efficient because it takes more Psycopg wraps the database server side cursor in named cursors . However, for immutable collections that are very large, or that are rarely accessed, I'm wondering if saving server side cursors in postgres would be a viable alternate caching strategy. Moving out-of-bound in a server-side cursor doesn’t result in an exception, if the backend doesn’t raise any (Postgres doesn’t tell us in a reliable way if we went out of bound). It was not obvious what was going wrong when I executed code similar to this test. QuestDB doesn’t support these cursors, but it supports so-called non-scrollable cursors, i. session. curso Each of the below mentioned solutions has its own cons. 3. The below code is creating a single csv with 2000 rows, but how can I create multiple csv files for every 2000 rows till the end of the This is caused by Django passing a WITH HOLD to its DECLARE CURSOR statement. orm. I'm trying to implement a server side cursor in order to "bypass" Django ORM weakness when it comes to fetch an huge amount of data from the database. postgresql 3 The psycopg2 module content19 client-side and server-side cursors, asynchronous communication and notifications, COPY support. DictCursor) While you certainly could insert a Python datetime into a row via psycopg2-- you would need to create a datetime object set to the current time, which can be done like this or via modules such as Delorean-- since you just want the current time, I would just leave that up to Postgres itself. rowcount is > 1. Modified 9 years, 7 months ago. result = cur. NamedTupleCursor, ): try: self. connect(db_uri_string) as conn: cursor = conn. It's not very helpful when working with large datasets, since the whole data is initially retrieved from DB into client-side memory and later chunked into separate frames based on Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company In your case the connector variable is a <class 'sqlalchemy. self. ) That is, something like this. We will see the performance of both cursors by inserting 1 As I understand it, if you are using a client side cursor, all the results are retrieved from the server when cursor. cursor('my_cursor') However, fetchall() will still return all rows at once. Client-side cursors# Client-side cursors are what Psycopg uses in its normal querying process. Use just. Using the name parameter on cursor() will create a ServerCursor or AsyncServerCursor, which can be used to retrieve partial results from a database. Changing server_side_binding value didn’t have any effect. You can just iterate over a named cursor. Use Please note Not naming the cursor in psycopg2 will cause the cursor to be client side as opposed to server side. connect("dbname='template1' host='localhost'") cur = conn. pandas. klin klin. There is an example of how to do what you want here Server side cursor in section:. tz (At a quick glance, server-side cursors can't be shared between connections, but I could be wrong there. 7. execute(query) for row in cursor: print row To use a returning cursor function execute it as usual: I have a Heroku app that uses a psycopg server-side cursor together with a LEFT JOIN query running on Heroku PG 13. Connection Pooling: Manage multiple connections efficiently using psycopg2. psycopg2 fetchmany vs named cursor. I use Python 3. It is easier to let psycopg2 do the server side cursor creation work just by naming it:. 1. pool. Many Python types are supported out-of-the-box and adapted to matching PostgreSQL data types; adaptation can be extended and How to use server side cursors with psycopg2. Session'> object. read_sql_query supports Python "generator" pattern when providing chunksize argument. Server-side cursors can usually scroll backwards only if declared scrollable. disable cursors: We'll lose benefits of server side cursors (chunked resultset). 121k 15 15 How to use server side cursors with psycopg2. def execute_query( self, query, query_params=None, cursor_factory=psycopg2. execute('select * from big_table') for row in stmt: #do something clever with the row Read more about Server side cursors. Otherwise it will be a regular client side cursor. Author: ryanbutterfield Posted: March 2, 2011 Language: Python Version: 1. 0. It returns None as per PEP-249:. I would test using subset of table and directly using psycopg2, not through Alembic. 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. psycopg2's cursors map to server-side cursors, so behaviour will correspond pretty well, but this isn't necessarily true of other drivers. I will document that server-side cursor don't behave like client-side one; specifically we just forward to 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. I'm using server side cursors and I want to set the search path to a specific schema. tz I'm comparing some of the features of Postgres clients for compatibility and I'm having difficulty getting prepared statements to work in psychopg2. PgJDBC for example receives the whole result set Performance Optimization: It is designed for high performance with features like server-side cursors and optimized query execution. The idea is that after having served a page in the middle of a collection "next" and "prev" links are much more likely to be used than a random query somewhere Fetch Records using a Server-Side Cursor. curso Having a server side cursor and fetching bunches of rows proved to be the most performant solution. This is a possible template to use: with conn. execute("SELECT * FROM my_data"); # Retrieve query results records=cur. cursor(name='cursor_x') query = "select * from t" cursor. is the client generating a cursor rather than passing sql. tz Server-side cursors can usually scroll backwards only if declared scrollable . 04 server (with libpq5 package installed). g. fxfjjb sjzd ruxd kfh hxztwkc zclh lxa tipzu azgdm vjyqbx