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Remove redundant _bt_truncate() comment paragraph.

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Peter Geoghegan 2019-09-12 09:51:27 -07:00
parent bc98e1ea64
commit 1b9becd43c
1 changed files with 8 additions and 13 deletions
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21
src/backend/access/nbtree/nbtutils.c
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@ -2092,25 +2092,20 @@ btbuildphasename(int64 phasenum)
* Caller's insertion scankey is used to compare the tuples; the scankey's
* argument values are not considered here.
*
* Sometimes this routine will return a new pivot tuple that takes up more
* space than firstright, because a new heap TID attribute had to be added to
* distinguish lastleft from firstright. This should only happen when the
* caller is in the process of splitting a leaf page that has many logical
* duplicates, where it's unavoidable.
*
* Note that returned tuple's t_tid offset will hold the number of attributes
* present, so the original item pointer offset is not represented. Caller
* should only change truncated tuple's downlink. Note also that truncated
* key attributes are treated as containing "minus infinity" values by
* _bt_compare().
*
* In the worst case (when a heap TID is appended) the size of the returned
* tuple is the size of the first right tuple plus an additional MAXALIGN()'d
* item pointer. This guarantee is important, since callers need to stay
* under the 1/3 of a page restriction on tuple size. If this routine is ever
* taught to truncate within an attribute/datum, it will need to avoid
* returning an enlarged tuple to caller when truncation + TOAST compression
* ends up enlarging the final datum.
* In the worst case (when a heap TID must be appended to distinguish lastleft
* from firstright), the size of the returned tuple is the size of firstright
* plus the size of an additional MAXALIGN()'d item pointer. This guarantee
* is important, since callers need to stay under the 1/3 of a page
* restriction on tuple size. If this routine is ever taught to truncate
* within an attribute/datum, it will need to avoid returning an enlarged
* tuple to caller when truncation + TOAST compression ends up enlarging the
* final datum.
*/
IndexTuple
_bt_truncate(Relation rel, IndexTuple lastleft, IndexTuple firstright,