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Adjust "root of to-be-deleted subtree" function.

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Restructure the function that locates the root of the to-be-deleted
subtree during nbtree page deletion.  Handle the conditions that make
page deletion unsafe in a slightly more uniform way, and acknowledge the
fact that the behavior with incomplete splits on internal pages is
different (as pointed out in the nbtree README as of commit 35bc0ec7).
Also invent new terminology that avoids ambiguity around which pages are
about to be deleted.  Consistently use the term "to-be-deleted subtree",
not the ambiguous term "branch".

We were calling the subtree parent page the "top parent page", but that
was quite misleading.  The top parent page usually refers to a page
unlinked from its siblings and marked deleted (during the second stage
of page deletion).  There was one kind of top parent page that we merely
removed a downlink from, and another kind of top parent page that we
actually marked deleted.  Eliminate the ambiguity by inventing a new
term ("subtree parent page") that refers to the former kind of page
only.
This commit is contained in:
Peter Geoghegan
2020-05-11 11:01:07 -07:00
parent a8be5364ac
commit 624686abcf
5 changed files with 278 additions and 215 deletions
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6
src/include/access/nbtxlog.h
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@@ -250,7 +250,7 @@ typedef struct xl_btree_vacuum
#define SizeOfBtreeVacuum (offsetof(xl_btree_vacuum, nupdated) + sizeof(uint16))
/*
* This is what we need to know about marking an empty branch for deletion.
* This is what we need to know about marking an empty subtree for deletion.
* The target identifies the tuple removed from the parent page (note that we
* remove this tuple's downlink and the *following* tuple's key). Note that
* the leaf page is empty, so we don't need to store its content --- it is
@@ -267,7 +267,7 @@ typedef struct xl_btree_mark_page_halfdead
BlockNumber leafblk; /* leaf block ultimately being deleted */
BlockNumber leftblk; /* leaf block's left sibling, if any */
BlockNumber rightblk; /* leaf block's right sibling */
BlockNumber topparent; /* topmost internal page in the branch */
BlockNumber topparent; /* topmost internal page in the subtree */
} xl_btree_mark_page_halfdead;
#define SizeOfBtreeMarkPageHalfDead (offsetof(xl_btree_mark_page_halfdead, topparent) + sizeof(BlockNumber))
@@ -294,7 +294,7 @@ typedef struct xl_btree_unlink_page
*/
BlockNumber leafleftsib;
BlockNumber leafrightsib;
BlockNumber topparent; /* next child down in the branch */
BlockNumber topparent; /* next child down in the subtree */
TransactionId btpo_xact; /* value of btpo.xact for use in recovery */
/* xl_btree_metadata FOLLOWS IF XLOG_BTREE_UNLINK_PAGE_META */