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I'd been wondering why I was sometimes seeing fractional rowcount estimates in parallel-query situations, and this seems to be the reason. (You won't see the fractional parts in EXPLAIN, because it prints rowcounts with %.0f, but they are apparent in the debugger.) A fractional rowcount is not any saner for a partial path than any other kind of path, and it's equally likely to break cost estimation for higher paths, so apply clamp_row_est() like we do in other places.
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/.