Horton

Re: [HACKERS] [WIP] Effective storage of duplicates in B-tree index.

Anastasia Lubennikova <a.lubennikova@postgrespro.ru>

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From: Anastasia Lubennikova <a.lubennikova@postgrespro.ru>
To: pgsql-hackers@lists.postgresql.org
Date: 2019-08-16T15:56:12Z
Lists: pgsql-hackers

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13.08.2019 18:45, Anastasia Lubennikova wrote:
>>   I also added a nearby FIXME comment to
>> _bt_insertonpg_in_posting() -- I don't think think that the code for
>> splitting a posting list in two is currently crash-safe.
>>
> Good catch. It seems, that I need to rearrange the code.
> I'll send updated patch this week.

Attached is v7.

In this version of the patch, I heavily refactored the code of insertion 
into
posting tuple. bt_split logic is quite complex, so I omitted a couple of
optimizations. They are mentioned in TODO comments.

Now the algorithm is the following:

- If bt_findinsertloc() found out that tuple belongs to existing posting 
tuple's
TID interval, it sets 'in_posting_offset' variable and passes it to
_bt_insertonpg()

- If 'in_posting_offset' is valid and origtup is valid,
merge our itup into origtup.

It can result in one tuple neworigtup, that must replace origtup; or two 
tuples:
neworigtup and newrighttup, if the result exceeds BTMaxItemSize,

- If two new tuple(s) fit into the old page, we're lucky.
call _bt_delete_and_insert(..., neworigtup, newrighttup, newitemoff) to
atomically replace oldtup with new tuple(s) and generate xlog record.

- In case page split is needed, pass both tuples to _bt_split().
  _bt_findsplitloc() is now aware of upcoming replacement of origtup with
neworigtup, so it uses correct item size where needed.

It seems that now all replace operations are crash-safe. The new patch 
passes
all regression tests, so I think it's ready for review again.

In the meantime, I'll run more stress-tests.

-- 
Anastasia Lubennikova
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company

Commits

  1. Teach pageinspect about nbtree deduplication.

  2. Doc: Fix deduplicate_items index term.

  3. Revise BTP_HAS_GARBAGE nbtree VACUUM comments.

  4. Remove unneeded "pin scan" nbtree VACUUM code.

  5. Cleanup code in reloptions.h regarding reloption handling

  6. Catch invalid typlens in a couple of places

  7. Compute XID horizon for page level index vacuum on primary.

  8. Make heap TID a tiebreaker nbtree index column.

  9. Avoid pin scan for replay of XLOG_BTREE_VACUUM in all cases

  10. Revert buggy optimization of index scans

  11. Restructure index access method API to hide most of it at the C level.

  12. Reduce pinning and buffer content locking for btree scans.

  13. Avoid scanning nulls at the beginning of a btree index scan.