How to UPSERT MERGE INSERT ON DUPLICATE UPDATE in PostgreSQL

Question

A very frequently asked question here is how to do an upsert  which is what MySQL calls INSERT     ON DUPLICATE UPDATE and the standard supports as part of the MERGE operation   Given that PostgreSQL doesn t support it directly  before pg 9 5   how do you do this  Consider the following   CREATE TABLE testtable       id integer PRIMARY KEY      somedata text NOT NULL     INSERT INTO testtable  id  somedata  VALUES  1   fred     2   bob      Now imagine that you want to  upsert  the tuples  2   Joe     3   Alan    so the new table contents would be    1   fred     2   Joe       -- Changed value of existing tuple  3   Alan      -- Added new tuple   That s what people are talking about when discussing an upsert  Crucially  any approach must be safe in the presence of multiple transactions working on the same table - either by using explicit locking  or otherwise defending against the resulting race conditions   This topic is discussed extensively at Insert  on duplicate update in PostgreSQL   but that s about alternatives to the MySQL syntax  and it s grown a fair bit of unrelated detail over time  I m working on definitive answers   These techniques are also useful for  insert if not exists  otherwise do nothing   i e   insert     on duplicate key ignore

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WITH UPD AS  UPDATE TEST TABLE SET SOME DATA    Joe  WHERE ID   2  RETURNING ID   INS AS  SELECT  2    Joe  WHERE NOT EXISTS  SELECT   FROM UPD   INSERT INTO TEST TABLE ID  SOME DATA  SELECT   FROM INS   Tested on Postgresql 9 3

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Here are some examples for insert     on conflict      pg 9 5         Insert  on conflict - do nothing     insert into dummy id  name  size  values 1   new name   3  on conflict do nothing      Insert  on conflict - do update  specify conflict target via column     insert into dummy id  name  size  values 1   new name   3  on conflict id  do update set name    new name   size   3     Insert  on conflict - do update  specify conflict target via constraint name     insert into dummy id  name  size  values 1   new name   3  on conflict on constraint dummy pkey do update set name    new name   size   4

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I am trying to contribute with another solution for the single insertion problem with the pre-9 5 versions of PostgreSQL  The idea is simply to try to perform first the insertion  and in case the record is already present  to update it   do    begin    insert into testtable id  somedata  values 2  Joe    exception when unique violation then   update testtable set somedata    Joe  where id   2  end       Note that this solution can be applied only if there are no deletions of rows of the table   I do not know about the efficiency of this solution  but it seems to me reasonable enough

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SQLAlchemy upsert for Postgres   9 5  Since the large post above covers many different SQL approaches for Postgres versions  not only non-9 5 as in the question   I would like to add how to do it in SQLAlchemy if you are using Postgres 9 5  Instead of implementing your own upsert  you can also use SQLAlchemy s functions  which were added in SQLAlchemy 1 1   Personally  I would recommend using these  if possible  Not only because of convenience  but also because it lets PostgreSQL handle any race conditions that might occur   Cross-posting from another answer I gave yesterday  https   stackoverflow com a 44395983 2156909   SQLAlchemy supports ON CONFLICT now with two methods on conflict do update   and on conflict do nothing     Copying from the documentation   from sqlalchemy dialects postgresql import insert  stmt   insert my table  values user email  a b com   data  inserted data   stmt   stmt on conflict do update      index elements  my table c user email       index where my table c user email like    gmail com        set  dict data stmt excluded data        conn execute stmt    http   docs sqlalchemy org en latest dialects postgresql html highlight conflict insert-on-conflict-upsert

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Since this question was closed  I m posting here for how you do it using SQLAlchemy  Via recursion  it retries a bulk insert or update to combat race conditions and validation errors   First the imports   import itertools as it  from functools import partial from operator import itemgetter  from sqlalchemy exc import IntegrityError from app import session from models import Posts   Now a couple helper functions  def chunk content  chunksize None          Groups data into chunks each with  at most   chunksize  items      https   stackoverflow com a 22919323 408556             if chunksize          i   iter content          generator    list it islice i  chunksize   for   in it count        else          generator   iter  content        return it takewhile bool  generator    def gen resources records          Yields a dictionary if the record s id already exists  a row object      otherwise              ids    item 0  for item in session query Posts id        for record in records          is row   hasattr record   to dict            if is row and record id in ids                It s a row but the id already exists  so we need to convert it                to a dict that updates the existing record  Since it is duplicate                also yield True             yield record to dict    True         elif is row                It s a row and the id doesn t exist  so no conversion needed                 Since it s not a duplicate  also yield False             yield record  False         elif record  id   in ids                It s a dict and the id already exists  so no conversion needed                 Since it is duplicate  also yield True             yield record  True         else                It s a dict and the id doesn t exist  so we need to convert it                 Since it s not a duplicate  also yield False             yield Posts   record   False   And finally the upsert function  def upsert data  chunksize None       for records in chunk data  chunksize           resources   gen resources records          sorted resources   sorted resources  key itemgetter 1            for dupe  group in it groupby sorted resources  itemgetter 1                items    g 0  for g in group               if dupe                   upsert   partial session bulk update mappings  Posts              else                   upsert   session add all              try                   upsert items                  session commit               except IntegrityError                    A record was added or deleted after we checked  so retry                                      modify accordingly by adding additional exceptions  e g                     except  IntegrityError  ValidationError  ValueError                  db session rollback                   upsert items              except Exception as e                    Some other error occurred so reduce chunksize to isolate the                    offending row s                  db session rollback                   num items   len items                   if num items  gt  1                      upsert items  num items    2                  else                      print  Error adding record     format items 0      Here s how you use it   gt  gt  gt  data               id   1   text    updated post1               id   5   text    updated post5               id   1000   text    new post1000          gt  gt  gt  upsert data    The advantage this has over bulk save objects is that it can handle relationships  error checking  etc on insert  unlike bulk operations

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9 5 and newer  PostgreSQL 9 5 and newer support INSERT     ON CONFLICT  key  DO UPDATE  and ON CONFLICT  key  DO NOTHING   i e  upsert  Comparison with ON DUPLICATE KEY UPDATE  Quick explanation  For usage see the manual - specifically the conflict action clause in the syntax diagram  and the explanatory text  Unlike the solutions for 9 4 and older that are given below  this feature works with multiple conflicting rows and it doesn t require exclusive locking or a retry loop  The commit adding the feature is here and the discussion around its development is here   If you re on 9 5 and don t need to be backward-compatible you can stop reading now   9 4 and older  PostgreSQL doesn t have any built-in UPSERT  or MERGE  facility  and doing it efficiently in the face of concurrent use is very difficult  This article discusses the problem in useful detail  In general you must choose between two options   Individual insert update operations in a retry loop  or Locking the table and doing batch merge  Individual row retry loop Using individual row upserts in a retry loop is the reasonable option if you want many connections concurrently trying to perform inserts  The PostgreSQL documentation contains a useful procedure that ll let you do this in a loop inside the database  It guards against lost updates and insert races  unlike most naive solutions  It will only work in READ COMMITTED mode and is only safe if it s the only thing you do in the transaction  though  The function won t work correctly if triggers or secondary unique keys cause unique violations  This strategy is very inefficient  Whenever practical you should queue up work and do a bulk upsert as described below instead  Many attempted solutions to this problem fail to consider rollbacks  so they result in incomplete updates  Two transactions race with each other  one of them successfully INSERTs  the other gets a duplicate key error and does an UPDATE instead  The UPDATE blocks waiting for the INSERT to rollback or commit  When it rolls back  the UPDATE condition re-check matches zero rows  so even though the UPDATE commits it hasn t actually done the upsert you expected  You have to check the result row counts and re-try where necessary  Some attempted solutions also fail to consider SELECT races  If you try the obvious and simple  -- THIS IS WRONG  DO NOT COPY IT  It s an EXAMPLE   BEGIN   UPDATE testtable SET somedata    blah  WHERE id   2   -- Remember  this is WRONG  Do NOT COPY IT   INSERT INTO testtable  id  somedata  SELECT 2   blah  WHERE NOT EXISTS  SELECT 1 FROM testtable WHERE testtable id   2    COMMIT   then when two run at once there are several failure modes  One is the already discussed issue with an update re-check  Another is where both UPDATE at the same time  matching zero rows and continuing  Then they both do the EXISTS test  which happens before the INSERT  Both get zero rows  so both do the INSERT  One fails with a duplicate key error  This is why you need a re-try loop  You might think that you can prevent duplicate key errors or lost updates with clever SQL  but you can t  You need to check row counts or handle duplicate key errors  depending on the chosen approach  and re-try  Please don t roll your own solution for this  Like with message queuing  it s probably wrong  Bulk upsert with lock Sometimes you want to do a bulk upsert  where you have a new data set that you want to merge into an older existing data set  This is vastly more efficient than individual row upserts and should be preferred whenever practical  In this case  you typically follow the following process   CREATE a TEMPORARY table  COPY or bulk-insert the new data into the temp table  LOCK the target table IN EXCLUSIVE MODE  This permits other transactions to SELECT  but not make any changes to the table   Do an UPDATE     FROM of existing records using the values in the temp table   Do an INSERT of rows that don t already exist in the target table   COMMIT  releasing the lock    For example  for the example given in the question  using multi-valued INSERT to populate the temp table  BEGIN   CREATE TEMPORARY TABLE newvals id integer  somedata text    INSERT INTO newvals id  somedata  VALUES  2   Joe     3   Alan     LOCK TABLE testtable IN EXCLUSIVE MODE   UPDATE testtable SET somedata   newvals somedata FROM newvals WHERE newvals id   testtable id   INSERT INTO testtable SELECT newvals id  newvals somedata FROM newvals LEFT OUTER JOIN testtable ON  testtable id   newvals id  WHERE testtable id IS NULL   COMMIT   Related reading  UPSERT wiki page UPSERTisms in Postgres Insert  on duplicate update in PostgreSQL  http   petereisentraut blogspot com 2010 05 merge-syntax html Upsert with a transaction Is SELECT or INSERT in a function prone to race conditions  SQL MERGE on the PostgreSQL wiki Most idiomatic way to implement UPSERT in Postgresql nowadays  What about MERGE  SQL-standard MERGE actually has poorly defined concurrency semantics and is not suitable for upserting without locking a table first  It s a really useful OLAP statement for data merging  but it s not actually a useful solution for concurrency-safe upsert  There s lots of advice to people using other DBMSes to use MERGE for upserts  but it s actually wrong  Other DBs   INSERT     ON DUPLICATE KEY UPDATE in MySQL MERGE from MS SQL Server  but see above about MERGE problems  MERGE from Oracle  but see above about MERGE problems

[postgresql] How to UPSERT (MERGE, INSERT ... ON DUPLICATE UPDATE) in PostgreSQL?

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