5 ms·

Cool, I’d love to know how this affects query performance, rather than just index size?

Skinney 6y ago
Roughly speaking: smaller index size = greater performance. The index also has to be read from disk (if not already in memory), which is faster the smaller it is. But also, once in memory, smaller size makes key-lookups or traversals faster. How much faster? It really depends on your index.
- petergeoghegan 6y ago
  B-Tree deduplication can also help to control duplicate index tuples caused by MVCC "version churn" -- including in unique indexes. See the paragraph that begins with "B-Tree indexes are not directly aware that under MVCC..." here: https://www.postgresql.org/docs/devel/btree-implementation.html#BTREE-DEDUPLICATION https://www.postgresql.org/docs/devel/btree-implementation.h...
mjevans 6y ago
If it helps either make tables small enough for RAM, or what used to be RAM small enough for cache, then it could help a LOT.
petergeoghegan 6y ago
It's hard to give a simple answer, but it's certainly possible for it to help performance significantly. Both for queries, and garbage collection by VACUUM. It's especially helpful with writes + low cardinality indexes.
redis_mlc 6y ago
Postgres has a write amplification problem - when you update or insert a row, it has to check all the indexes to see what was touched (unlike MySQL.) Uber made this famous in a blog post where they migrated off postgres because of it. So with Postgres I try to keep the number of indexes per table down to around 5, but with MySQL InnoDB, 20 indexes is not a problem for many use cases. The other little-known Postgres problem is that by default, installing it sets up replication that writes a local file in a busy manner. This can cause problems with fsck or on low-end SSD/flash storage resulting in corruption that causes postgres restarts to fail hard, so disable that.
- andy_ppp 6y ago
  I’m interested to find out why the community feel this answer is incorrect. I don’t know enough about database internals to know! Would be good if someone could help me to become more informed about this. I can’t personally see a way that indexes wouldn’t be touched when inserts happen in MySQL but who knows?
  - jchb 6y ago
    Previously on HN: Why Uber Engineering Switched from Postgres to MySQL (2016): https://news.ycombinator.com/item?id=17280239 https://news.ycombinator.com/item?id=17280239 A PostgreSQL response to Uber [pdf]: https://news.ycombinator.com/item?id=14222721 https://news.ycombinator.com/item?id=14222721 One important point is that Uber wrote this running on Postgres 9.2. The latest Postgres release is now 12.4
    - andy_ppp 6y ago
      Thanks, I just always trust Postgres more from my experiences. Reading some of the above I’m fairly unconvinced by Uber’s reasoning...
      - pgaddict 6y ago
        I'd say some of the limitations hit by Uber were real, and were a bit of a PITA even before that. Some were improved significantly since then, others still need to be considered when designing application accessing PostgreSQL. I'd say it was a nice example what happens when you design application based on how you imagine an ideal database would work, not realizing that there's a lot of complexity and limitations. And then when you eventually have to face reality you blame the database for having to do trade-offs and refuse to mitigate the trouble by changing the application.
      - takeda 6y ago
        All jobs that I had where MySQL was used had some kind of issues. First time there was a database that one day just died and refused to start. When investigating, it turns that some tables got corrupted (fortunately, the scan tool that Congress with MySQL was able to fix it, but I also encountered databases which were FUBARed). After checking the machine, it had over 100 days off uptime (so it wasn't abrupt shutdown), the disk also had plenty of free space, so the corruption didn't happen because of disk being full. So MySQL just broke itself out of nowhere. Another time (I think it was a MariaDB) the database get corrupted every few days. First we though it was hardware, so we replaced disks, then the entire machine. But it continued to happen. Eventually it turned out it was a bug and with specific pattern written to the disk, when MariaDB read that block back it incorrectly thought the block was encrypted and discarded it. Updating to a version that fixed it, responded the issue, but that's really a nasty bug that caused data corruption. Postgresql on the other hand with my experience always was nice even in bad situations. When we had network issue (due to failed switch) one the problem was fixed the database reconnected back and trained replication, MySQL in similar situation required to tell it to reconnect again, some even setting up replication again (which requires restoring backup on slave and reconfiguring replication again, which required few hours). In another instance we had main postgresql set up replication to a DR site. The database data was stored on filesystem provided by NetApp. A consultant had a task to deprecate filesystems that were no longer used, so he actually put them offline for 30 days, before deleting them. Around 20 days in we noticed that the master was running out of space. Turns out the disk was utilized by unshipped logs to the standby. Once filesystem at the DR site was put back online everything resolved all by itself.
dspillett 6y ago
Like SQL Server's row-level and page-level compression, you get more rows per page, so need to read less pages to process the query. From cold this means less disk or network IO loading the pages from storage into RAM, and it means you can fit more into RAM (and potentially CPU cache when spinning over small enough chunks of data) so you can do the same with less RAM or reduce IO so be faster with the same RAM allocation (as pages will not need to be evicted as often to make room for others). The trade-off is a bit of CPU time, every time the page is referenced. Not just when it is updated or read from storage, but potentially every time it is accessed, even just reading, as the pages held in RAM are in the same format as on disk so the decompression needs to happen on read also (unless postgres keeps a cache of pre-decompressed pages to help with this but my gut suggests that would not be worth the complication - where it would help significantly you would just not use this technique anyway). In a great many scenarios, I would say most, this trade-off is worth it because of the relative cost of IO and CPU time needed to save some of it. It might be significantly detrimental to performance in situations where enough of your data fits into RAM anyway and you are performing CPU-heavy analysis upon it. Side benefits include reducing the size of your backups and reducing the time needed to take them, so there are other potential infrastructure and administration savings to be had too.
- petergeoghegan 6y ago
  There is no overhead for reads, since deduplication doesn't compress the data as such. Rather, deduplication creates a representation that removes duplicates but is otherwise identical to the existing representation.

Roughly speaking: smaller index size = greater performance. The index also has to be read from disk (if not already in memory), which is faster the smaller it is. But also, once in memory, smaller size makes key-lookups or traversals faster. How much faster? It really depends on your index.

petergeoghegan 6y ago
B-Tree deduplication can also help to control duplicate index tuples caused by MVCC "version churn" -- including in unique indexes. See the paragraph that begins with "B-Tree indexes are not directly aware that under MVCC..." here: https://www.postgresql.org/docs/devel/btree-implementation.html#BTREE-DEDUPLICATION https://www.postgresql.org/docs/devel/btree-implementation.h...

mjevans 6y ago

If it helps either make tables small enough for RAM, or what used to be RAM small enough for cache, then it could help a LOT.

petergeoghegan 6y ago

It's hard to give a simple answer, but it's certainly possible for it to help performance significantly. Both for queries, and garbage collection by VACUUM. It's especially helpful with writes + low cardinality indexes.

redis_mlc 6y ago

Postgres has a write amplification problem - when you update or insert a row, it has to check all the indexes to see what was touched (unlike MySQL.) Uber made this famous in a blog post where they migrated off postgres because of it. So with Postgres I try to keep the number of indexes per table down to around 5, but with MySQL InnoDB, 20 indexes is not a problem for many use cases. The other little-known Postgres problem is that by default, installing it sets up replication that writes a local file in a busy manner. This can cause problems with fsck or on low-end SSD/flash storage resulting in corruption that causes postgres restarts to fail hard, so disable that.

andy_ppp 6y ago
I’m interested to find out why the community feel this answer is incorrect. I don’t know enough about database internals to know! Would be good if someone could help me to become more informed about this. I can’t personally see a way that indexes wouldn’t be touched when inserts happen in MySQL but who knows?
- jchb 6y ago
  Previously on HN: Why Uber Engineering Switched from Postgres to MySQL (2016): https://news.ycombinator.com/item?id=17280239 https://news.ycombinator.com/item?id=17280239 A PostgreSQL response to Uber [pdf]: https://news.ycombinator.com/item?id=14222721 https://news.ycombinator.com/item?id=14222721 One important point is that Uber wrote this running on Postgres 9.2. The latest Postgres release is now 12.4
  - andy_ppp 6y ago
    Thanks, I just always trust Postgres more from my experiences. Reading some of the above I’m fairly unconvinced by Uber’s reasoning...
    - pgaddict 6y ago
      I'd say some of the limitations hit by Uber were real, and were a bit of a PITA even before that. Some were improved significantly since then, others still need to be considered when designing application accessing PostgreSQL. I'd say it was a nice example what happens when you design application based on how you imagine an ideal database would work, not realizing that there's a lot of complexity and limitations. And then when you eventually have to face reality you blame the database for having to do trade-offs and refuse to mitigate the trouble by changing the application.
    - takeda 6y ago
      All jobs that I had where MySQL was used had some kind of issues. First time there was a database that one day just died and refused to start. When investigating, it turns that some tables got corrupted (fortunately, the scan tool that Congress with MySQL was able to fix it, but I also encountered databases which were FUBARed). After checking the machine, it had over 100 days off uptime (so it wasn't abrupt shutdown), the disk also had plenty of free space, so the corruption didn't happen because of disk being full. So MySQL just broke itself out of nowhere. Another time (I think it was a MariaDB) the database get corrupted every few days. First we though it was hardware, so we replaced disks, then the entire machine. But it continued to happen. Eventually it turned out it was a bug and with specific pattern written to the disk, when MariaDB read that block back it incorrectly thought the block was encrypted and discarded it. Updating to a version that fixed it, responded the issue, but that's really a nasty bug that caused data corruption. Postgresql on the other hand with my experience always was nice even in bad situations. When we had network issue (due to failed switch) one the problem was fixed the database reconnected back and trained replication, MySQL in similar situation required to tell it to reconnect again, some even setting up replication again (which requires restoring backup on slave and reconfiguring replication again, which required few hours). In another instance we had main postgresql set up replication to a DR site. The database data was stored on filesystem provided by NetApp. A consultant had a task to deprecate filesystems that were no longer used, so he actually put them offline for 30 days, before deleting them. Around 20 days in we noticed that the master was running out of space. Turns out the disk was utilized by unshipped logs to the standby. Once filesystem at the DR site was put back online everything resolved all by itself.

dspillett 6y ago

Like SQL Server's row-level and page-level compression, you get more rows per page, so need to read less pages to process the query. From cold this means less disk or network IO loading the pages from storage into RAM, and it means you can fit more into RAM (and potentially CPU cache when spinning over small enough chunks of data) so you can do the same with less RAM or reduce IO so be faster with the same RAM allocation (as pages will not need to be evicted as often to make room for others). The trade-off is a bit of CPU time, every time the page is referenced. Not just when it is updated or read from storage, but potentially every time it is accessed, even just reading, as the pages held in RAM are in the same format as on disk so the decompression needs to happen on read also (unless postgres keeps a cache of pre-decompressed pages to help with this but my gut suggests that would not be worth the complication - where it would help significantly you would just not use this technique anyway). In a great many scenarios, I would say most, this trade-off is worth it because of the relative cost of IO and CPU time needed to save some of it. It might be significantly detrimental to performance in situations where enough of your data fits into RAM anyway and you are performing CPU-heavy analysis upon it. Side benefits include reducing the size of your backups and reducing the time needed to take them, so there are other potential infrastructure and administration savings to be had too.

petergeoghegan 6y ago
There is no overhead for reads, since deduplication doesn't compress the data as such. Rather, deduplication creates a representation that removes duplicates but is otherwise identical to the existing representation.