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/
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