Examples include MySQL's INSERT...ON DUPLICATE KEY UPDATE, or VoltDB's UPSERT statement.The absence of this fea… This happens to insulate ExecUpdate() from havingto care about "invisible tuples", which are now possible (although westill throw an error, just with a useful error message that phrasesthe problem in reference to this new feature). After a long time of waiting, PostgreSQL 9.5 introduced INSERT ON CONFLICT [DO UPDATE] [DO NOTHING]. If might not be, since _bt_checkunique() looks at laterB-Tree pages (the value locked page is merely "the first leaf page thevalue could be on"). Postgresql, update if row with some unique value exists, else insert , This newly option has two varieties: INSERT ON CONFLICT DO UPDATE: If record matched, it is updated with the new data value. Given therestrictions necessarily imposed on this pseudo-rescanning(principally the outright rejection of anything that necessitatesPARAM_EXEC parameters during planning), this is safe, as far as I'maware. INSERT ON Introduction to the PostgreSQL upsert. Here is a table of key, value pairs: demo=# SELECT * FROM kv; key | value -----+----- host | 127.0.0.1 port | 5432 (2 rows) A common use case is to insert a row only if it does not exist – and if it does, do not overwrite. PostgreSQL › PostgreSQL - general. I have (temporarily) hacked theoptimizer to prevent index-only scans, which are problematic here, byadding disable_cost when a query parse tree that uses the feature isseen. Of course, at higher isolation levels serialization errors are thrownwhen something inconsistent with the higher level's guarantees wouldotherwise need to occur (even for the IGNORE variant). If the index used in ON CONFLICT() is a partial index, predicates of the index (WHERE …) must be added after the ON CONFLICT clause. Once you buy into the idea that we must avoidholding on to "value locks" of whatever form - as Heikki evidently did- then exactly what form they take is ultimately only a detail.Granted, it's a very important detail, but a detail nonetheless. Compatibility INSERT conforms to the SQL standard, except that the RETURNING clause is a PostgreSQL extension, as is the ability to use WITH with INSERT, and the ability to specify an alternative action with ON CONFLICT. Second, specify columns and their new values after SET keyword. That makes it impossible for the heap tuple slot to fail torelate to the tuple from the B-Tree, that is under consideration forlocking/updating. Any expression using the table's columns, and/or columns of other tables mentioned in FROM, can be computed. As I mentioned, I have incorporated feedback from Kevin Grittner. All of the following SQLqueries are valid with the patch applied: -- Nesting within wCTE:WITH t AS ( INSERT INTO z SELECT i, 'insert' FROM generate_series(0, 16) i ON CONFLICT UPDATE SET v = v || 'update' -- use ofoperators/functions in targetlist RETURNING * -- only projects inserted tuples, never updated tuples)SELECT * FROM t JOIN y ON t.k = y.a ORDER BY a, k; -- IGNORE variant:INSERT INTO upsert(key, val) VALUES(1, 'insert') ON CONFLICT IGNORE; -- predicate within UPDATE auxiliary statement (row is still lockedwhen the UPDATE predicate isn't satisfied):INSERT INTO upsert(key, val) VALUES(1, 'insert') ON CONFLICT UPDATEWHERE val != 'delete'; As with SQL MERGE (at least as implemented in other systems),subqueries may not appear within the UPDATE's targetlist, nor may theyappear within the special WHERE clause. Once data has been added to a database, the SQL UPDATE command can be used to modify the column values in the rows of a table. Also, some of the restrictions that I already mentioned- on updatable views, inheritance, and foreign tables - are probablyunnecessary. This allows INSERT statements toperform UPSERT operations (if you want a more formal definition ofUPSERT, I refer you to my pgCon talk's slides [1], or the thread inwhich I delineated the differences between SQL MERGE and UPSERT [2]).The patch builds on previous work in this area, and incorporatesfeedback from Kevin and Andres. Basically, I want to be comfortableabout my rationale for it being okay that a "non-MVCC" "index scan"doesn't hold a pin, but right now I'm not. As in previous incarnations, we lock each tuple (although, of course,only with the UPDATE variant). In other words: I'm relying on the way VACUUM actually works toprevent premature garbage collection. This is onthe same dedicated 8 core server, with plenty of concurrency. I'm not sure how widely appreciated this pointis, but I believe that Heikki appreciates it. This is probably too paranoid, though: the fact that the upserter'stransaction is running ought to imply that GetOldestXmin() returns anXID sufficient to prevent this. Having taken into account the experience of myself and Heikki, andall that is implied by taking that approach ***while avoidingunprincipled deadlocks***, you continue to believe that an approachbased on speculative heap insertion, or some alternative scheme isbetter than what I have done to the nbtree code here, or you otherwisedislike something about the proposed value locking scheme. And so, the predicate is considered once, afterconclusively locking a conflict tuple. This post continues to dive deeper into the topic. We may have to interlock against vacuum by sitting on theB-Tree buffer pin (but not the value lock) throughout locking +update. But if we do,EvalPlanQual() is (once the tuple is locked) only ever evaluated on aconclusively committed and locked-by-us conflict tuple as part of theprocess of updating, even though it's possible for the UPDATEpredicate to be satisfied where conceivably it would not be satisfiedby the tuple version actually visible to the command's MVCC snapshot.I think this is the correct behavior. How this new approach works (Executor and Optimizer stuff)============================================. The Right Thing is far fromobvious, and there is very little to garner from other systems, sinceSQL MERGE promises essentially nothing about concurrency, both asspecified by the standard and in practice. Okay, I think I manage to solve it by adding balance = excluded.balance inside the parenthesis of ON CONFLICT clause. This lets application developers write less code and do more work in SQL. But this is aparticularly unsympathetic case, because I've deliberately exaggeratedthe effects of heavyweight lock contention on leaf pages by using aserial primary key. On Thu, May 10, 2018 at 12:07 PM, Adrian Klaver. This is an open item. I thought, only the columns that I declared inside the ON CONFLICT()  parenthesis can be called in SET. However, unlike with that SQLite feature, CONFLICT onlyrefers to a would-be duplicate violation, and not a violation of anyother kind of constraint. A day before yesterday, I got an email like, does it require to add a unique index on those columns which we require to be in ON CONFLICT clause? In general,at least with Postgres it's entirely possible that values rejecteddiffer from the values appearing in the VALUES() list, due to theeffects of before triggers. Upsert (INSERT ON CONFLICT DO) is a new function of PostgreSQL 9.5. Use multiple conflict_target in ON. I've also triedto break up the discussion usefully (the question of how everythingfits together at a high level can hopefully be discussed separatelyfrom the question of how "value locks" are actually implemented). Postgres will insert a record if it doesn’t exist, or it will update that particular record if it already does exist. Old versions ofthis patch used to sit on the B-Tree buffer pin to prevent concurrentdeletion only as long as value locks were held, but maybe it isn'tgood enough to sit on the pin until before we lock/update, as valuelocks are released: dropping the pin implies that the heap tuple canphysically go away, and in general the same TID may then containanything. He recognized the need to be able to easily *release* valuelocks, so as to avoid "unprincipled deadlocks", where under highconcurrency there are deadlocks between sessions that only UPSERT asingle row at a time. However, it ismore or less independently planned, and entirely driven by the INSERTModifyTable. If you are using an earlier version, you will need a workaround to have the upsert feature. They allow you to do something like this: INSERT INTO upsert (key, val) VALUES (1 'val') ON DUPLICATE KEY UPDATEval = VALUES(val); The implication is that the updated value comes from the INSERT'sVALUES() list, but emulating that seems like a bad idea. Here's what we are going to talk about: Therefore, it seems worth considering thepossibility that the nbtree README's observations on the necessity ofholding a pin to interlock against VACUUM (for non-MVCC snapshots)apply. transformStmt()/transformUpdateStmt() is called, and isinsulated from having to care about the feature as a special case).There are some restrictions on what this auxiliary update may do, butFWIW there are considerably fewer than those that the equivalent MySQLor SQLite feature imposes on their users. The new (post-update) values of the table's columns are used. PostgreSQL › PostgreSQL - hackers. Although what I've done is a temporary kludge, the basic idea offorcing a particular type of relation scan has a precedent: UPDATEWHERE CURRENT OF artificially forces a TID scan, because only a TIDscan will work correctly there. ON CONSTRAINT constraint_name – where the constraint name could be the name of … I thought that I went too long without posting something about all ofthis to the list to get feedback, and so I decided to post this WIPpatch set. It is likely worthwhile to teach theoptimizer that we really don't care about how the one and only baserel within the UPDATE auxiliary subquery (the target table) isscanned, if only to save a few cycles. A candidate row will only be inserted if that row does not violate any unique constraints. This can be revisited.). the plpsqlthing that the docs suggest). You may write the DML statementlike this: INSERT INTO upsert(key, val) VALUES(1, 'insert') ON CONFLICT WITHINupsert_pkey UPDATE SET val = 'update'; I think that there is a good chance that at least some people willwant to make this mandatory. When we are inserting a new row into a particular table, the PostgreSQL will upgrade the row if it is already present, or else, it will add the new row. Search everywhere only in this topic Advanced Search . This modified text is an extract of the original Stack Overflow Documentation created by following contributors and released under CC BY-SA 3.0 I think it makes sense to deal with it a bit later. But the "INSERT part" of thequery has no additional limitations, so you may for example putsubqueries within a VALUES() clause, or INSERT...SELECT...ON CONFLICTUPDATE... just as you'd expect. If anyone finds my (virtually unchanged) page heavyweight lock basedvalue locking approach objectionable, I ask that the criticism beframed in a way that makes a sharp distinction between each of thefollowing: 1. Previously, we have to use upsert or merge statement to do this kind of operation. It's possible to imagine a worldin which HeapTupleSatisfiesVacuum() is smart enough to realize thatthe tuple UPSERT wants to lock is not visible to anyone (assuming MVCCsemantics, etc), and never can be. PostgreSQL 9.5: Insert IF not Exists, Update IF Exists (Insert ON CONFLICT option) In above article, I used only one single key in ON CONFLICT clause. INSERT ... ON CONFLICT DO UPDATE with _any_ constraint ‹ Previous Topic Next Topic › Classic List: Threaded ♦ ♦ 26 messages 1 2. Yes, that's what I figured out eventually. there is no ExecModifyTable() call in respect of this newauxiliary ModifyTable plan). We’ve been talking about offline-first with Hasura and RxDB (essentially Postgres and PouchDB underneath).. With "equivalent" upserts, it's about ~66,000 TPS. Just forexample, the unprincipled deadlocks test case that illustrated theproblem with early "promise tuple" style approaches to value locking[6] involved only a single unique index. Afterall, it wasn't as if we could abort a subxact to release locks, whichis what the "UPSERT looping subxact" pattern does. Perhaps we can come up with amore tasteful syntax that covers all interesting cases (consider theissues with partial unique indexes and before triggers for example,where a conclusion reached about which index to use during parseanalysis may subsequently be invalidated by user-defined code, orambiguous specifications in the face of overlapping attributes betweentwo unique composite indexes, etc). I was conflicted on whetheror not I should include the "unpin later" logic at all; for now I'veleft it in, if only as a placeholder. In this article, we’ll take a closer look at the PostgreSQL UPSERT keyword and check out some examples of its use. I have also published an article on it. Is anyone in this camp? insert into p values (4, 'a') on conflict (a) do update set b = excluded.b; postgres=# insert into p values (4, 'b') on conflict (a) do update set b = excluded.b; ERROR: attribute number 3 exceeds number of columns 2 I attach my patch here for your reference, which I polished … OTOH, I'm not sure that there existsanything that looks like a precedent for relying on blocking vacuum inthis manner, and it might turn out to be limiting to rely on this.And, I hasten to add, my fix (sitting on a B-Tree pin throughout rowlocking) is in another way perhaps not paranoid enough: Who is to saythat our conflicting value is on the same B-Tree leaf page as ourvalue lock? My bad. It is a discussion and guide to implementing CouchDB style conflict resolution with Postgres (central backend database) and PouchDB (frontend app user database).. The optional RETURNING clause causes UPDATE to compute and return value (s) based on each row actually updated. So, for example, during parseanalysis, UPDATE transformation occurs in an ad-hoc fashion tightlydriven by the parent INSERT, but using the existing infrastructure(i.e. During the execution of the parent ModifyTable, a special auxiliarysubquery (the UPDATE ModifyTable) is considered as a special case.This is not a subplan of the ModifyTable node in the conventionalsense, and so does not appear within EXPLAIN output. Starting with version 9.5, PostgreSQL allows “upserts” (update or insert) of rows into a table via the ON CONFLICT clause of the INSERT statement. You don't accept that value locks must be easily released in theevent of a conflict. In addition, stresstesting is an important part of my testing strategy. Since we can (I believe) talk about the value locking aspect and therest of the patch independently, we should do so...unless you're incamp 1, in which case I guess that we'll have to thrash it out. We all seem to be in agreementthat we should update at READ COMMITTED if *no* version of the tupleis visible. There is a newExecUpdate() call site within ExecLockUpdateTuple(). The effect is similar to MySQL: INSERT INTO customers (id, first_name, last_name, email) VALUES (30797, 'hooopo1', 'wang', '[email protected]') ON CONFLICT(id) DO UPDATE SET first_name = EXCLUDED.first_name, last_name = EXCLUDED.last_name; Batch Upsert. Upserts are comparedagainst "equivalent" inserts when we know we'll never update, andagainst "equivalent" updates when we know we'll never insert. It feels natural and appropriate to me that if the special UPDATE qualisn't satisfied, we still lock the tuple. On Thu, May 10, 2018 at 5:57 PM, Alban Hertroys. The performance of the patch seems quite good, and is something thatthese stress-testing bash scripts also test. Search everywhere only in this topic Advanced Search. INSERT has been augmented with a newclause, but that clause does not unreasonably fail to play nice withany other aspect of insertion. Caused by: org.postgresql.util.PSQLException: ERROR: duplicate key value violates unique constraint […] The PostgresSQL INSERT doc u mentation specifies an ON CONFLICT … From the PostgreSQL wiki, MERGE is typically used to merge two tables, and was introduced in the 2003 SQL standard. Patch extends the INSERT statement, adding a new ONCONFLICT { UPDATE | IGNORE clause... The syntax for … the optional RETURNING clause causes UPDATE to compute and return value ( )... '' upserts, it consists of one or more records in a table hackers request. Postgres and PouchDB underneath ) tupleis visible … the optional RETURNING clause causes UPDATE to and. 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