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Previously, the way this worked was that a parallel pg_dump would re-order the TABLE_DATA items in the dump's TOC into decreasing size order, and separately re-order (some of) the INDEX items into decreasing size order. Then pg_dump would dump the items in that order. Later, parallel pg_restore just followed the TOC order. This method had lots of deficiencies: * TOC ordering randomly differed between parallel and non-parallel dumps, and was hard to predict in the former case, causing problems for building stable pg_dump test cases. * Parallel restore only followed a well-chosen order if the dump had been done in parallel; in particular, this never happened for restore from custom-format dumps. * The best order for restore isn't necessarily the same as for dump, and it's not really static either because of locking considerations. * TABLE_DATA and INDEX items aren't the only things that might take a lot of work during restore. Scheduling was particularly stupid for the BLOBS item, which might require lots of work during dump as well as restore, but was left to the end in either case. This patch removes the logic that changed the TOC order, fixing the test instability problem. Instead, we sort the parallelizable items just before processing them during a parallel dump. Independently of that, parallel restore prioritizes the ready-to-execute tasks based on the size of the underlying table. In the case of dependent tasks such as index, constraint, or foreign key creation, the largest relevant table is used as the metric for estimating the task length. (This is pretty crude, but it should be enough to avoid the case we want to avoid, which is ending the run with just a few large tasks such that we can't make use of all N workers.) Patch by me, responding to a complaint from Peter Eisentraut, who also reviewed the patch. Discussion: https://postgr.es/m/5137fe12-d0a2-4971-61b6-eb4e7e8875f8@2ndquadrant.com
PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: https://www.postgresql.org/download See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.