On Thu, Jan 6, 2022 at 6:45 AM Vik Fearing <v...@postgresfriends.org> wrote: > > On 1/5/22 11:03 PM, Corey Huinker wrote: > > > > There was similar work being done for system periods, which are a bit > > simpler but require a side (history) table to be created. > > This is false. SYSTEM_TIME periods do not need any kind of history. > This was one of the problems I had with Surafel's attempt because it was > confusing the period with SYSTEM VERSIONING. Versioning needs the > period but the inverse is not true.
This is an interesting point. Syntactically, there are three different things: the generated started/end columns, the period declaration, and the WITH SYSTEM VERSIONING modifier to the table. You could declare a system period without making the table versioned. Practically speaking I don't know why you'd ever create a system period without a versioned table (do you know of any uses Vik?), but perhaps we can exploit the separation to add system periods in the same patch that adds application periods. The first two bits of syntax *are* tied together: you need columns with GENERATED ALWAYS AS ROW START/END to declare the system period, and less intuitively the standard says you can't use AS ROW START/END unless those columns appear in a system period (2.e.v.2 under Part 2: Foundation, 11.3 <table definition>). Personally I'd be willing to ignore that latter requirement. For one thing, what does Postgres do with the columns if you drop the period? Dropping the columns altogether seems very harsh, so I guess you'd just remove the GENERATED clause. Another weird thing is that you don't (can't) say STORED for those columns. But they are certainly stored somewhere. I would store the values just like any other column (even if non-current rows get moved to a separate table). Also then you don't have to do anything extra when the GENERATED clause is dropped. If we wanted to support system-time periods without building all of system versioning, what would that look like? At first I thought it would be a trivial addition to part-1 of the patch here, but the more I think about it the more it seems to deserve its own patch. One rule I think we should follow is that using a non-system-versioned table (with a system period) should get you to the same place as using a system-versioned table and then removing the system versioning. But the standard says that dropping system versioning should automatically drop all historical records (2 under Part 2: Foundation, 11.30 <drop system versioning clause>). That actually makes sense though: when you do DML we automatically update the start/end columns, but we don't save copies of the previous data (and incidentally the end column will always be the max value.) So there is a use case, albeit a thin one: you get a Rails-like updated_at column that is maintained automatically by your RDBMS. That is pretty easy, but I think I'd still break it out into a separate patch. I'm happy to work on that as something that builds on top of my part-1 patch here. Yours, Paul
