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. -- Álvaro Herrera https://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
