On Sat, Sep 28, 2019 at 9:22 PM Alvaro Herrera <alvhe...@2ndquadrant.com> wrote: > > On 2019-Sep-28, Bruce Momjian wrote: > > > The CREATE INDEX docs already say: > > > > In a concurrent index build, the index is actually entered into > > the system catalogs in one transaction, then two table scans occur in > > two more transactions. Before each table scan, the index build must > > wait for existing transactions that have modified the table to > > terminate. > > After the second scan, the index build must wait for any transactions > > --> that have a snapshot (see <xref linkend="mvcc"/>) predating the second > > --> scan to terminate. Then finally the index can be marked ready for use, > > > > So, having multiple concurrent index scans is just a special case of > > having to "wait for any transactions that have a snapshot", no? I am > > not sure adding a doc mention of other index builds really is helpful. > > I always thought that create index concurrently was prevented from > running concurrently in a table by the ShareUpdateExclusive lock that's > held during the operation.
You mean multiple CICs on a single table at the same time? Yes, that (unfortunately) isn't possible, but I'm concerned in the patch with the fact that CIC on table X blocks CIC on table Y. James
