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doc PG 18 relnotes: add GROUP BY column elimination item

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`@@ -284,6 +284,23 @@ This optimization can be disabled using enable_distinct_reordering.`
`284`	`284`	`</para>`
`285`	`285`	`</listitem>`
`286`	`286`
	`287`	`+<!--`
	`288`	`+Author: David Rowley <drowley@postgresql.org>`
	`289`	`+2024-12-12 [bd10ec529] Detect redundant GROUP BY columns using UNIQUE indexes`
	`290`	`+-->`
	`291`	`+`
	`292`	`+<listitem>`
	`293`	`+<para>`
	`294`	`+Ignore GROUP BY columns that are functionally dependent on other columns (Zhang Mingli, Jian He, David Rowley)`
	`295`	`+<ulink url="&commit_baseurl;bd10ec529">§</ulink>`
	`296`	`+</para>`
	`297`	`+`
	`298`	`+<para>`
	`299`	`+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`
	`300`	`+from the grouping. This was already true for non-deferred primary keys.`
	`301`	`+</para>`
	`302`	`+</listitem>`
	`303`	`+`
`287`	`304`	`<!--`
`288`	`305`	`Author: Richard Guo <rguo@postgresql.org>`
`289`	`306`	`2024-10-09 [67a54b9e8] Allow pushdown of HAVING clauses with grouping sets`

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