80558c1f5a
Add a new flag, consider_parallel, to each RelOptInfo, indicating whether a plan for that relation could conceivably be run inside of a parallel worker. Right now, we're pretty conservative: for example, it might be possible to defer applying a parallel-restricted qual in a worker, and later do it in the leader, but right now we just don't try to parallelize access to that relation. That's probably the right decision in most cases, anyway. Using the new flag, generate parallel sequential scan plans for plain baserels, meaning that we now have parallel sequential scan in PostgreSQL. The logic here is pretty unsophisticated right now: the costing model probably isn't right in detail, and we can't push joins beneath Gather nodes, so the number of plans that can actually benefit from this is pretty limited right now. Lots more work is needed. Nevertheless, it seems time to enable this functionality so that all this code can actually be tested easily by users and developers. Note that, if you wish to test this functionality, it will be necessary to set max_parallel_degree to a value greater than the default of 0. Once a few more loose ends have been tidied up here, we might want to consider changing the default value of this GUC, but I'm leaving it alone for now. Along the way, fix a bug in cost_gather: the previous coding thought that a Gather node's transfer overhead should be costed on the basis of the relation size rather than the number of tuples that actually need to be passed off to the leader. Patch by me, reviewed in earlier versions by Amit Kapila. |
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README
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: http://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 http://www.postgresql.org/download/. For more information look at our web site located at http://www.postgresql.org/.