pgsql: Doc: add detail about EXPLAIN's "Disabled" property

Doc: add detail about EXPLAIN's "Disabled" property

c01743aa4 and later 161320b4b adjusted the EXPLAIN output to show which
plan nodes were chosen despite being disabled by the various enable*
GUCs. Prior to e22253467, the disabledness of a node was only evident by
a large startup cost penalty. Since we now explicitly tag disabled nodes
with a boolean property in EXPLAIN, let's add some documentation to
provide some details about why and when disabled nodes can appear in the
plan.

Author: Laurenz Albe, David Rowley
Discussion: https://postgr.es/m/883729e429267214753d5e438c82c73a58c3db5d.camel@cybertec.at

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/84b8fccbe5c21cc2a05f8cf91903cadc6ebfe680

Modified Files
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doc/src/sgml/perform.sgml | 25 +++++++++++++++++++++++++
1 file changed, 25 insertions(+)