Thread: GIN indexed unique constraint?
Hi,
Is it possible to declare a UNIQUE constraint that uses GIN indexing?
If so, what would the definition look like?
Allan.
On Sun, Jun 27, 2021 at 11:18 AM Allan Kamau <kamauallan@gmail.com> wrote:
Is it possible to declare a UNIQUE constraint that uses GIN indexing?
Doesn't seem to be possible. The btree_gin extension would provide the necessary code but it states explicitly that:
"... and they lack one major feature of the standard B-tree code: the ability to enforce uniqueness."
"David G. Johnston" <david.g.johnston@gmail.com> writes:
> On Sun, Jun 27, 2021 at 11:18 AM Allan Kamau <kamauallan@gmail.com> wrote:
>> Is it possible to declare a UNIQUE constraint that uses GIN indexing?
> Doesn't seem to be possible. The btree_gin extension would provide the
> necessary code but it states explicitly that:
> "... and they lack one major feature of the standard B-tree code: the
> ability to enforce uniqueness."
GIN stores all the component elements of its input values separately.
It'd be tremendously hard even to identify which inputs share exactly
the same component elements; let alone whether inputs sharing the
same elements should be considered "equal". For example, ARRAY[1,2]
and ARRAY[2,1] would give rise to identical sets of index entries in
a GIN array_ops index.
In short, no, this isn't something you do with a GIN index.
regards, tom lane
On Sun, Jun 27, 2021 at 9:41 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
"David G. Johnston" <david.g.johnston@gmail.com> writes:
> On Sun, Jun 27, 2021 at 11:18 AM Allan Kamau <kamauallan@gmail.com> wrote:
>> Is it possible to declare a UNIQUE constraint that uses GIN indexing?
> Doesn't seem to be possible. The btree_gin extension would provide the
> necessary code but it states explicitly that:
> "... and they lack one major feature of the standard B-tree code: the
> ability to enforce uniqueness."
GIN stores all the component elements of its input values separately.
It'd be tremendously hard even to identify which inputs share exactly
the same component elements; let alone whether inputs sharing the
same elements should be considered "equal". For example, ARRAY[1,2]
and ARRAY[2,1] would give rise to identical sets of index entries in
a GIN array_ops index.
In short, no, this isn't something you do with a GIN index.
regards, tom lane
Thank you David and Tom for your speedy and informative responses.
Allan.