The only scalable delete in Postgres is DROP TABLE

https://planetscale.com/blog/the-only-scalable-delete

by hollylawly • 1 month ago

196 80 1 month ago

sgarland4• 1 month ago

The same is true to a lesser extent in MySQL / MariaDB. It does better since it doesn’t do oldest-to-newest tuple chains, but it’s still adding non-trivial work to the DB, much of which is effectively wasted if you don’t care about the visibility of the deleted (or soon-to-be deleted) tuples to other transactions.

I sincerely hope that Planetscale’s efforts succeed long-term to shift devs’ understanding and acceptance of RDBMS operations. Their blog posts and docs are generally quite good. IME, devs (and even ops-ish teams) simply do not care about all of this, and will create elaborate bespoke tooling to run DELETEs in bulk, because they either don’t understand the capabilities of the database, or don’t want to deal with the [minor] increased complexity that a partitioned schema brings, and will happily pay the extra cost / latency for deletions.

kro• 1 month ago

mysql/maria also lets you turn off/down the isolation level for queries if you know the guarantees aren't needed, to speed things up. I think postgres does not have that option.

alternbet25676• 1 month ago

Postgres does support changing the isolation level at query, session and config: https://www.postgresql.org/docs/current/sql-set-transaction....

pstuart• 1 month ago

Yep, partitions are the way to go there.

awinter-py• 1 month ago

^ this

been exploring clickhouse and while it is definitely not a general purpose DB, for time-series shaped data that can survive some insert latency, the automatic partition-based TTL is very nice and, at least so far, requires zero attention to maintain

which I guess is solved by `pg_partman` at the bottom of the post

crazygringo5• 1 month ago

Only by a weird definition of "scalable". The first sentence says:

Counterintuitively, large DELETEs add work to the database.

There is nothing counterintuitive about this. It takes just as much work to delete a row as it takes to insert a row. Why wouldn't it? Obviously you have to do almost all the same operations: write a log, write the deletion, update indices, replicate it, etc.

And yes, it's a well-known trick for all major relational databases (not just Postgres) that if you want to delete 90% of rows from a large table, it's much faster to just copy the rows you want to keep to a new table, run DROP TABLE on the old table, and rename the new table to the old table. Since DROP TABLE is ~instantaneous, mainly involving table-level metadata.

DELETE scales just fine, in the sense that if you are constantly inserting and deleting individual rows, DELETE scales the same as INSERT.

Basic database functionality is designed around the assumption of lots of small transactions. Whenever you have to do something involving millions of rows at once, you generally need to investigate solutions that work well in "bulk". E.g. loading rows directly from a file rather than with SQL, adding indices only after the data has been loaded rather than before, disabling foreign key checks on large operations (if you know by design that the keys are valid)... and yes, taking advantage of DROP TABLE instead of DELETE. This doesn't mean small transactions aren't scalable, it just means bulk operations are qualitatively different and benefit from their own solutions. And DELETE is no different from INSERT in this regard.

echoangle• 1 month ago

And yes, it's a well-known trick for all major relational databases (not just Postgres) that if you want to delete 90% of rows from a large a table, it's much faster to just copy the rows you want to keep to a new table, run DROP TABLE on the old table, and rename the new table to the old table.

Dumb question but why does the optimizer not just do that in secret then? Seems like something that should be detectable with some heuristics.

mordae• 1 month ago

It drops dependents.

sgarland4• 1 month ago

I assume partly because that would be extremely surprising behavior, and depending on the RDBMS and version, could introduce unexpected stalls. For example, MySQL < 8.0.23 scans the entire buffer pool to clear pages that were dropped, which can take a long time on large instances. There is / was a similar issue with its adaptive hash index, which AFAIK wasn’t ever fixed, though AHI’s default being shifted to OFF in 8.4 is a workaround, in a very hacky way.

Retr0id2• 1 month ago

Maintaining the expected observable behaviours would get complicated if queries (especially other updates) against the same table are happening concurrently.

crazygringo5• 1 month ago

Because what do you do if rows are being inserted in the original table, while the new table is having rows copied over? You'll get missing rows.

You can only do the DROP TABLE trick if you know nothing else is writing to the table at the same time. You know if that's the case, according to your business logic. The database has no idea.

The DROP TABLE trick effectively bypasses all the normal guarantees of data consistency. This is why it's so fast. But you have to know that that's a safe thing to do for your data.

layer8• 1 month ago

Because dropping a table effectively requires an exclusive lock on the table during that whole operation, affecting parallel transactions.

setr• 1 month ago

It takes just as much work to delete a row as it takes to insert a row. Why wouldn't it? Obviously you have to do almost all the same operations: write a log, write the deletion, update indices, replicate it, etc.

It takes far more work to delete/update than insert. My recent example is updating ~2TB of text data was about 40x slower than inserting 12TB (was trying to correct some large text truncation that occurred during migration into PG, ended up being faster to redo).

crazygringo5• 1 month ago

It takes far more work to delete/update than insert.

Updating rows of text data is going to be more work, because variable-length text can't be updated in-place. So in terms of allocating space, it's more like a delete plus an insert. That's not surprising. (An in-place update that doesn't touch indices is generally going to be faster than an insert, though.)

I'm not aware of instances where a delete is "far more work" than an equivalent insert though. That's not the general case, and I'm having a hard time thinking of any situations where that would be true.

winterbloom• 1 month ago

how does that solution work if the table that is dropped has foreign key constraints?

crazygringo5• 1 month ago

That's why I said:

disabling foreign key checks on large operations

And you have to know that, according to your business logic, what you're doing is safe.

Retr0id2• 1 month ago

if you are constantly inserting and deleting individual rows, DELETE scales the same as INSERT

Technically correct, but for a small table with a high churn rate, the performance characteristics may be surprising in that the "n" in most big-O calculations includes all inserts since the last VACUUM, not the actual number of resident rows.

coldtea• 1 month ago

There is nothing counterintuitive about this. It takes just as much work to delete a row as it takes to insert a row. Why wouldn't it?

Because e.g. DROP also effectively deletes the rows but takes way way less work.

umpalumpaaa• 1 month ago

Does this “Drop hack” work well with foreign keys, triggers and constraints?

globular-toast• 1 month ago

It takes just as much work to delete a row as it takes to insert a row. Why wouldn't it?

Because your data structure/algorithm supports fast deletes? File systems support deleting entire directories instantly. I'm not aware of any fundamental reason why DELETE in a SQL database must take as long as an insert?

silversmith• 1 month ago

I usually don't like the "read the article" replies, but in this case it's warranted - the second section explains how multiple transactions interact there, and the "deleted" record still needs to actually exist for older transactions.

crazygringo5• 1 month ago

I suggest you read the following sentence of my comment, the one after your quote. It provides your answer.

And file systems deleting huge directories instantly is the equivalent of DROP TABLE here, which I also mention in my comment.

dpark• 1 month ago

File systems support deleting entire directories instantly.

What file system supports this? I’ve never experienced the joy of deleting an entire large directory and seeing it disappear instantly.

I’m not sure how this would work with any modern file system that supports basic notions like hard links.

drob518• 1 month ago

Yea, this seems as obvious to me as why it’s more efficient to reformat a drive partition than to delete every file that might be stored there. Or why it’s more efficient to free a whole memory arena than to free every single memory block allocated within it. If you know you’re throwing everything away, it’s more efficient to invoke a “throw it all away” action than to throw away each piece individually.

jandrewrogers• 1 month ago

This generalizes to most (all?) databases. Selective deletion is largely an unsolved problem at scale in databases to the extent it doesn't release the deleted resources. Under the hood databases try to turn this into selective resource truncation, which scales much better, but in most cases that is not possible without careful design of your data model.

Similarly, you often have to remind devs that in many databases an UPDATE is just an INSERT + DELETE, with all of the scaling issues implied.

mike_hearn2• 1 month ago

RocksDB and other LSM tree backed databases do have cheap deletes and updates, although you could argue that's because they make everything else expensive. If you have spare cores it can be a good trade though.

levkk• 1 month ago

CRUD apps don't usually delete in bulk. It's also hard to structure partitions in a way that doesn't wipe out months of important business data -- this is why teams often ETL their DB into Snowflake/ClickHouse and only then drop partitions. That makes it hard for the app to use that data again.

The better approach is either to change your storage engine (e.g. OrioleDB is working on adding the undo log to Pg), or to shard which distributes the vacuum load across multiple servers.

sgarland4• 1 month ago

They should be performing bulk deletions, due to GDPR: “Data must be stored for the shortest time possible.” Unless you have some kind of rolling cron checking every few minutes (and even then, depending on your scale, that may well be considered bulk), that generally resolves to something like daily or weekly deletions.

aftbit• 1 month ago

TIL about pg_partman

https://github.com/pgpartman/pg_partman/blob/development/doc...

saisrirampur3• 1 month ago

Partially true but too much of a blanket statement and clickbaity.

DELETE with well-tuned autovacuum works pretty well. Have seen it work at TBs scale with no hicuups. If DELETEs are large, we used to recommend customers to follow that with a manual VACUUM for table to reclaim space right away for future rows.

DROP TABLE can be risky, it requires an ACCESS EXCLUSIVE LOCK and if its waiting, it blocks all other statements following it, because of how lock queues work in Postgres. And you cannot keep doing high concurrent DROP TABLEs to run your large scale CRUD app.

CharlieDigital• 1 month ago

    > And you cannot keep doing high concurrent DROP TABLEs to run your large scale CRUD app

In this kind of use case/design, I would assume it would make use of partitions to make this more palatable in which case it would seem that you would bypass this issue of "high concurrent DROP TABLE". Large scale CRUD app just points to recent-ish partitions. Old partitions are either going to be low or on access and can be dropped easily or transformed/transferred into some long term/cold storage.

saisrirampur3• 1 month ago

(Most) CRUD/OLTP applications don't delete data by timestamp; they delete by primary key. For those workloads, DROP TABLE (or dropping a partition) isn't a palatable option.

The entire premise here is really about time-series workloads where most operations are based on a timestamp. In those apps partition dropping has been a standard and recommended retention strategy for years. That's precisely why extensions like pg_partman and TimescaleDB exist. Given that context, the title feels more clickbaity, and could easily mislead readers into thinking this applies broadly to OLTP systems when it doesn't;

saisrirampur3• 1 month ago

Separately, this is one of the Postgres autovacuum tuning blog that I've ever read. Have seen it work across many customers and it is also simple to decipher and implement. https://www.citusdata.com/blog/2022/07/28/debugging-postgres...

smithcheck4• 1 month ago

I have been using TRUNCATE allm the way, and is fast as very good.

xenator• 1 month ago

Deleting whole file is faster then deleting rows in file.

foreigner• 1 month ago

Surprisingly to remove small numbers of rows in multiple tables (e.g. cleanup between automated tests), DELETE is often faster than TRUNCATE! It's counterintuitive but just measure it for yourself and see. Note you can DELETE from multiple tables in one statement using CTEs, and that way you don't need to think about foreign key dependency order.

twotwotwo• 1 month ago

Years ago work was bit by the analogous thing in MySQL. Like it usually does, it took a chain of events:

- We wrote a cronjob to periodically DELETE for a retention policy on a table we'd just created. Most senior person on the team reviewed it, looked fine.

- Unusually for us, we prioritize QA'ing a different feature for release, delaying the release of this cronjob and a bunch of other code.

- During that delay, the new table accumulated many times more rows to be deleted than we'd expected during review.

- Release happens. All looks well since the initial delete wasn't a migration and cronjob hasn't run yet; engineer doing the release signs off.

- Cronjob runs, deleting hundreds of millions of rows quickly.

- Next day, replica lag's high and MySQL's transaction history is very high. MySQL keeps transaction history around until purge threads have visited all the affected pages on disk.

- The bad cluster conditions last for days and lead to other problems.

This omits detail and the 'noise' of everything else we were watching. But it gets across how the code and MySQL behaved.

Like most exciting events, it led to multiple changes to avoid a repeat. For retention policies, our new approach was one at the end of PlanetScale's post, to partition and drop old partitions. Transitioning to this from a huge unpartitioned table can be fun!

If a table is append-only and already huge, with lots of rows already past the retention threshold, you might only copy the rows to be kept to the new partitioned table: copy what you can, lock tables, do a last catch-up copy and swap tables. (Roughly the blog's 'performant one-off delete'.)

If the table's merely kind of big, gh-ost or such could allow you to ALTER without causing lag, locking, etc.

At a scale below that, you could run a slow incremental 'nibble' delete while watching server stats, and a step below that, plain ALTERs or DELETEs are fine.

Using partitioning has fun bits, too. In MySQL, the partition key has to be part of any unique index, understandably. But you have to keep that in mind when you're using INSERT..ON DUPLICATE KEY UPDATE and relying on uniqueness to trigger the update. Things stay interesting!

I hear Vitess shops like PlanetScale usually don't run multi-terabyte myqsld instances in the first place: even when physical nodes are big, they run many smaller mysqlds on them. That wouldn't make all this fully irrelevant--huge deletes would still sometimes be worse than copy-swap-drop--but it does seem real handy for taming issues that tend to worsen with mysqld size, like replication lag. All to say, little bit jelly of their setup over there!

DarkUranium3• 1 month ago

One thing I did a while ago was to make deletes part of inserts, to amortize the cost.

The main reason was to avoid a separate cron job, but it had other benefits (and downsides) too. Something like:

    DELETE FROM foo WHERE expires_at < now() LIMIT 10;
    INSERT INTO foo .....;

Note the LIMIT: it ensures the latency stays under control even if we've suddenly hit 50k rows that need deleting.

And by deleting (up to) 10 each time we insert one, it ensures obsolete things will eventually get deleted.

Obviously, this isn't viable when the deletions must happen due to strict policy (e.g. legal compliance) since it can't ensure when things get deleted, just that they eventually do. IIRC, in my case, I used it for a password reset tokens table. There's no legal issue there and keeping expired ones around is fine as long as the code also checks `expires_at` to make sure it's still valid (which would be a good practice regardless, for defense in depth).

silvestrov• 1 month ago

Problem is that PostgreSQL does not support LIMIT on the DELETE command.

I have no idea why, it seems such an obvious feature for supporting large databases.

inigyou• 1 month ago

This website appears to be hard blocking my IP address (drops all packets( so I can't read the article.

cowthulhu• 1 month ago

IMO, needing to clear out an entire table is an indicator that something has gone wrong with your design.

Don't get me wrong, I've definitely done it before, but it's in the same bucket as VACUUM for me... high impact interventions used to fix a mistake I made, not "course of business" actions.

baq2• 1 month ago

You should run vacuum as often as possible in Postgres if you’re doing anything other than INSERTs, this is a design tradeoff in Postgres itself. It’s the reason autovacuum exists and why tuning it is so important for performance; nothing wrong with doing a VACUUM ANALYZE after finishing a large DML batch job.

TurdF3rguson2• 1 month ago

what does as often as possible mean? It will auto-vacuum when it's idle, right? Why not just let it do that?

saltcured• 1 month ago

DROP DATABASE, for when a bunch of calls to DROP TABLE seems like too much overhead...

mike_hock• 1 month ago

pg_dropcluster for when a bunch of calls to DROP DATABASE seems like too much overhead.

flossly• 1 month ago

Years ago I heard Oracle's db had an edge on PG when it comes to DELETEs. I guess that's still the case...

mike_hearn2• 1 month ago

Oracle has ALTER TABLE [name] MOVE ONLINE INCLUDING ROWS [predicates]. This is basically what you want - it keeps only the data matching the predicate, it's efficient and it works without disrupting the app. That might be the edge you heard about.

asgeirn• 1 month ago

Also ALTER TABLE DROP PARTITION

ankitml• 1 month ago

This is directionally correct approach. Deleting a large chunk of rows, in a large table does lead to unpredictable-bad behaviour for a while until those dead tuples are handled.

I have used a very similar strategy by forking repack client https://github.com/reorg/pg_repack/pull/326 This works out of the box with rds/cloudsql etc.

buremba• 1 month ago

Materialized tables are useful for time-series or sharding-like use-cases. You essentially offload the work to INSERT time to locate the data into relevant buckets/sub-tables that you can DROP later.

We use materialized views for append-only timeseries data for https://lobu.ai and the retention policies define how we DROP the tables so we don't DELETE/UPDATE any rows in the tables.

The long term storage is Iceberg on S3 that's ingested via Postgresql replication, suitable for OLAP use-cases. Postgresql only stores the dimensional OLTP data the users can update and the hot append-only event data.

Syzygies• 1 month ago

I can't believe I'm the first to Rick roll this thread with the most famous XKCD comic of all time:

https://xkcd.com/327/

TurdF3rguson2• 1 month ago

I can because this thread is not about sql injection.

missingdays• 1 month ago

But there's DROP TABLE in both the title and the comic, so it must be relevant

znnajdla• 1 month ago

Why are databases so hard?

DarkUranium3• 1 month ago

Because of the guarantees they provide.

Storing some data in a binary file isn't very hard. Making it so that you can do quick lookups on it (indexes) and implementing joins in a sane way is kinda hard, but easy compared to the real problem:

Ensuring ACID (in the case of "traditional" databases). I.e. Atomicity, Consistency, Isolation, Durability.

You need to protect against data corruption in the event of failure, all while guaranteeing atomic operations at the user level concurrently (in most production DBs; SQLite is a notable exception in that it fully serializes writes --- but it can get away with this because it's an embedded database with the primary use case of a single-process writer). And the entire thing must land on a known good state at the end of all of those concurrent transactions.

... and they must do it all while maintaining good performance, and sometimes on a combination of filesystem + hardware that's actively hostile towards the idea of data integrity (e.g. hidden RAM caches in disk or RAID controllers that don't flush on power loss --- thankfully, those are getting rarer, or so I've come to understand).

insumanth• 1 month ago

This mostly applies to almost any database.

Deletes are Writes and Writes are resource intensive. This is more prominent on databases like Elastic Search.

When I was tasked to delete millions of (old) documents, it overloaded the cluster and almost brought it down. Only scalable solution was to split the index and drop the whole index.

elnerd• 1 month ago

I’m using Postgres for my DNS log service. I only store data for 90 days. To delete data, my strategy is to use partitions based on month. At the start of every month, I drop one partition.

I am not sure of this is the best way to do this, but it works for me.

nicman23• 1 month ago

funnily enough i had a broken mysql / mariadb install that drop would not work at all - pinned a core to 100% - but delete would be instant in 1M rows lmao

HackerThemAll• 1 month ago

Is that post written by people who think a degree in IT is a waste of time, or is it written for just those people?

One day you'll maybe discover ACID properties of RDBMS systems, and all the puzzles will fall into place.

lanycrost• 1 month ago

but partitioning not adding it's own overheads in DTO/Models side? and what about the performance versus just drops. If it's not for space saving you can just update and add delete marker or only do inserts and use the latest one.

mugiseyebrows• 1 month ago

update users set deleted=1 where

venkat971• 1 month ago

Associating drop table to scalabe delete is not a good framing. These are two different things, and very sensitive on production workloads if there are FK and MVs involved. Databases has 'Trucate table' for a reason. The post header should have used 'Truncate table' instead of 'Drop table' framing.

The only scalable delete in Postgres is DROP TABLE

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