On 6/20/23 20:08, jian he wrote: > On Wed, Jun 21, 2023 at 12:25 AM Tomas Vondra > ... >> http://www.wiscorp.com/sqlmultisets.zip > >> Conceptually, a multiset is an unordered collection of elements, all of the >> same type, with dupli- >> cates permitted. Unlike arrays, a multiset is an unbounded collection, with >> no declared maximum >> cardinality. This does not mean that the user can insert elements in a >> multiset without limit, just >> that the standard does not mandate that there should be a limit. This is >> analogous to tables, which >> have no declared maximum number of rows. > > Postgres arrays don't have size limits.
Right. You can say int[5] but we don't enforce that limit (I haven't checked why, but presumably because we had arrays before the standard existed, and it was more like a list in LISP or something.) > unordered means no need to use subscript? Yeah - there's no obvious way to subscript the items when there's no implicit ordering. > So multiset is a more limited array type? > Yes and no - both are collection types, so there are similarities and differences. Multiset does not need to keep the ordering, so in this sense it's a relaxed version of array. > null is fine. but personally I feel like so far the hashset main > feature is the quickly aggregate unique value using hashset. > I found using hashset count distinct (non null values) is quite faster. True. That's related to fast membership checks. regards -- Tomas Vondra EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
