Re: Patch: Global Unique Index

 ---- On Thu, 24 Nov 2022 08:00:59 -0700  Thomas Kellerer  wrote --- 
 > Pavel Stehule schrieb am 24.11.2022 um 07:03:
 > >     There are many Oracle users that find global indexes useful despite
 > >     their disadvantages.
 > >
 > >     I have seen this mostly when the goal was to get the benefits of
 > >     partition pruning at runtime which turned the full table scan (=Seq Scan)
 > >     on huge tables to partition scans on much smaller partitions.
 > >     Partition wise joins were also helpful for query performance.
 > >     The substantially slower drop partition performance was accepted in thos cases
 > >
 > >
 > >     I think it would be nice to have the option in Postgres as well.
 > >
 > >     I do agree however, that the global index should not be created automatically.
 > >
 > >     Something like CREATE GLOBAL [UNIQUE] INDEX ... would be a lot better
 > >
 > >
 > > Is it necessary to use special marks like GLOBAL if this index will
 > > be partitioned, and uniqueness will be ensured by repeated
 > > evaluations?
 > >
 > > Or you think so there should be really forced one relation based
 > > index?
 > >
 > > I can imagine a unique index on partitions without a special mark,
 > > that will be partitioned,  and a second variant classic index created
 > > over a partitioned table, that will be marked as GLOBAL.
 > 
 > 
 > My personal opinion is, that a global index should never be created
 > automatically.
 > 
 > The user should consciously decide on using a feature
 > that might have a serious impact on performance in some areas.


Agreed, if a unique index is created on non-partition key columns without including the special mark (partition key columns), it may be a mistake from user. (At least I make this mistake all the time). Current PG will give you a warning to include the partition keys, which is good. 

If we were to automatically turn that into a global unique index, user may be using the feature without knowing and experiencing some performance impacts (to account for extra uniqueness check in all partitions).