Optimize update of tables with generated columns

When updating a table row with generated columns, we only need to
recompute those generated columns whose base columns have changed in
this update and keep the rest unchanged. This can result in a
significant performance benefit (easy to reproduce for example with a
tsvector column). The required information was already kept in
RangeTblEntry.extraUpdatedCols; we just have to make use of it.

A small problem is that right now ExecSimpleRelationUpdate() does not
populate extraUpdatedCols. That needs fixing first. This is also
related to the issue discussed in "logical replication does not fire
per-column triggers"[0]. I'll leave my patch here while that issue is
being resolved.

[0]:
https://www.postgresql.org/message-id/flat/21673e2d-597c-6afe-637e-e8b10425b240%402ndquadrant.com

--
Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services