doc PG 18 relnotes: add GROUP BY column elimination item

With a nod to PG 9.6.

Reported-by: jian he
Discussion: https://postgr.es/m/CACJufxEqs=EXZETwtaOooTFhZrtxvSWg8M2uPfzjNtS3wQ6Dzw@mail.gmail.com

diff --git a/doc/src/sgml/release-18.sgml b/doc/src/sgml/release-18.sgml
index 66cdeb67961b33b9d135d71d95f75da871ad6c94..33e9199d758a6634da38cbf02329f4863738b8a7 100644 (file)
--- a/doc/src/sgml/release-18.sgml
+++ b/doc/src/sgml/release-18.sgml
@@ -284,6 +284,23 @@ This optimization can be disabled using enable_distinct_reordering.
 
 
 
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+Ignore GROUP BY columns that are functionally dependent on other columns (Zhang Mingli, Jian He, David Rowley)
+§
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+If a GROUP BY clause includes all columns of a unique index, as well as other columns of the same table, those other columns are redundant and can be dropped
+from the grouping.  This was already true for non-deferred primary keys.
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