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doc: 1-byte varlena headers can be used for user PLAIN storage

This also updates some C comments.Reported-by: suchithjn22@gmail.comDiscussion:https://postgr.es/m/167336599095.2667301.15497893107226841625@wrigleys.postgresql.orgAuthor: Laurenz Albe (doc patch)Backpatch-through: 11

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Lines changed: 1 addition & 3 deletions

Original file line number	Diff line number	Diff line change
`@@ -456,9 +456,7 @@ for storing <acronym>TOAST</acronym>-able columns on disk:`
`456`	`456`	`<listitem>`
`457`	`457`	`<para>`
`458`	`458`	`<literal>PLAIN</literal> prevents either compression or`
`459`		`- out-of-line storage; furthermore it disables use of single-byte headers`
`460`		`- for varlena types.`
`461`		`- This is the only possible strategy for`
	`459`	`+ out-of-line storage. This is the only possible strategy for`
`462`	`460`	`columns of non-<acronym>TOAST</acronym>-able data types.`
`463`	`461`	`</para>`
`464`	`462`	`</listitem>`

Lines changed: 10 additions & 1 deletion

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`@@ -68,7 +68,16 @@`
`68`	`68`	`#include"utils/memutils.h"`
`69`	`69`
`70`	`70`
`71`		`-/* Does att's datatype allow packing into the 1-byte-header varlena format? */`
	`71`	`+/*`
	`72`	`+ * Does att's datatype allow packing into the 1-byte-header varlena format?`
	`73`	`+ * While functions that use TupleDescAttr() and assign attstorage =`
	`74`	`+ * TYPSTORAGE_PLAIN cannot use packed varlena headers, functions that call`
	`75`	`+ * TupleDescInitEntry() use typeForm->typstorage (TYPSTORAGE_EXTENDED) and`
	`76`	`+ * can use packed varlena headers, e.g.:`
	`77`	`+ * CREATE TABLE test(a VARCHAR(10000) STORAGE PLAIN);`
	`78`	`+ * INSERT INTO test VALUES (repeat('A',10));`
	`79`	`+ * This can be verified with pageinspect.`
	`80`	`+ */`
`72`	`81`	`#defineATT_IS_PACKABLE(att) \`
`73`	`82`	`((att)->attlen == -1 && (att)->attstorage != TYPSTORAGE_PLAIN)`
`74`	`83`	`/* Use this if it's already known varlena */`

Lines changed: 2 additions & 1 deletion

Original file line number	Diff line number	Diff line change
`@@ -2513,7 +2513,8 @@ range_contains_elem_internal(TypeCacheEntry typcache, const RangeType r, Datum`
`2513`	`2513`	`* values into a range object. They are modeled after heaptuple.c's`
`2514`	`2514`	`* heap_compute_data_size() and heap_fill_tuple(), but we need not handle`
`2515`	`2515`	`* null values here. TYPE_IS_PACKABLE must test the same conditions as`
`2516`		`- * heaptuple.c's ATT_IS_PACKABLE macro.`
	`2516`	`+ * heaptuple.c's ATT_IS_PACKABLE macro. See the comments thare for more`
	`2517`	`+ * details.`
`2517`	`2518`	`*/`
`2518`	`2519`
`2519`	`2520`	`/* Does datatype allow packing into the 1-byte-header varlena format? */`

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