Hi, This patch has a CF status of "Needs Review" [1], but it seems there were CFbot test failures last time it was run [2]. Please have a look and post an updated version if necessary.
Attached is v4 of the statistics export/import patch.
This version has been refactored to match the design feedback received previously.
The system views are gone. These were mostly there to serve as a baseline for what an export query would look like. That role is temporarily reassigned to pg_export_stats.c, but hopefully they will be integrated into pg_dump in the next version. The regression test also contains the version of each query suitable for the current server version.
The export format is far closer to the raw format of pg_statistic and pg_statistic_ext_data, respectively. This format involves exporting oid values for types, collations, operators, and attributes - values which are specific to the server they were created on. To make sense of those values, a subset of the columns of pg_type, pg_attribute, pg_collation, and pg_operator are exported as well, which allows pg_import_rel_stats() and pg_import_ext_stats() to reconstitute the data structure as it existed on the old server, and adapt it to the modern structure and local schema objects.
pg_import_rel_stats matches up local columns with the exported stats by column name, not attnum. This allows for stats to be imported when columns have been dropped, added, or reordered.
pg_import_ext_stats can also handle column reordering, though it currently would get confused by changes in expressions that maintain the same result data type. I'm not yet brave enough to handle importing nodetrees, nor do I think it's wise to try. I think we'd be better off validating that the destination extended stats object is identical in structure, and to fail the import of that one object if it isn't perfect.