Re: WIP: Covering + unique indexes.

From: Anastasia Lubennikova <a(dot)lubennikova(at)postgrespro(dot)ru>
To: Peter Geoghegan <pg(at)heroku(dot)com>, David Steele <david(at)pgmasters(dot)net>
Cc: Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, PostgreSQL mailing lists <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: WIP: Covering + unique indexes.
Date: 2016-03-28 13:45:28
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21.03.2016 19:53, Anastasia Lubennikova:
> 19.03.2016 08:00, Peter Geoghegan:
>> On Fri, Mar 18, 2016 at 5:15 AM, David Steele <david(at)pgmasters(dot)net>
>> wrote:
>>> It looks like this patch should be marked "needs review" and I have
>>> done so.
>> Uh, no it shouldn't. I've posted an extensive review on the original
>> design thread. See CF entry:
>> Marked "Waiting on Author".
> Thanks to David,
> I've missed these letters at first.
> I'll answer here.
>> * You truncate (remove suffix attributes -- the "included" attributes)
>> within _bt_insertonpg():
>> - right_item = CopyIndexTuple(item);
>> + indnatts = IndexRelationGetNumberOfAttributes(rel);
>> + indnkeyatts = IndexRelationGetNumberOfKeyAttributes(rel);
>> +
>> + if (indnatts != indnkeyatts)
>> + {
>> + right_item = index_reform_tuple(rel, item, indnatts,
>> indnkeyatts);
>> + right_item_sz = IndexTupleDSize(*right_item);
>> + right_item_sz = MAXALIGN(right_item_sz);
>> + }
>> + else
>> + right_item = CopyIndexTuple(item);
>> ItemPointerSet(&(right_item->t_tid), rbkno, P_HIKEY);
>> I suggest that you do this within _bt_insert_parent(), instead, iff
>> the original target page is know to be a leaf page. That's where it
>> needs to happen for conventional suffix truncation, which has special
>> considerations when determining which attributes are safe to truncate
>> (or even which byte in the first distinguishing attribute it is okay
>> to truncate past)
> I agree that _bt_insertonpg() is not right for truncation.
> Furthermore, I've noticed that all internal keys are solely the copies
> of "High keys" from the leaf pages. Which is pretty logical.
> Therefore, if we have already truncated the tuple, when it became a
> High key, we do not need the same truncation within
> _bt_insert_parent() or any other function.
> So the only thing to worry about is the HighKey truncation. I rewrote
> the code. Now only _bt_split cares about truncation.
> It's a bit more complicated to add it into index creation algorithm.
> There's a trick with a "high key".
> /*
> * We copy the last item on the page into the new page, and then
> * rearrange the old page so that the 'last item' becomes its
> high key
> * rather than a true data item. There had better be at least
> two
> * items on the page already, else the page would be empty of
> useful
> * data.
> */
> /*
> * Move 'last' into the high key position on opage
> */
> To be consistent with other steps of algorithm ( all high keys must be
> truncated tuples), I had to update this high key on place:
> delete the old one, and insert truncated high key.
> The very same logic I use to truncate posting list of a compressed
> tuple in the "btree_compression" patch. [1]
> I hope, both patches will be accepted, and then I'll thoroughly merge
> them .
>> * I think the comparison logic may have a bug.
>> Does this work with amcheck? Maybe it works with bt_index_check(), but
>> not bt_index_parent_check()? I think that you need to make sure that
>> _bt_compare() knows about this, too. That's because it isn't good
>> enough to let a truncated internal IndexTuple compare equal to a
>> scankey when non-truncated attributes are equal.
> It is a very important issue. But I don't think it's a bug there.
> I've read amcheck sources thoroughly and found that the problem
> appears at
> "invariant_key_less_than_equal_nontarget_offset()
> static bool
> invariant_key_less_than_equal_nontarget_offset(BtreeCheckState *state,
> Page nontarget, ScanKey
> key,
> OffsetNumber upperbound)
> {
> int16 natts = state->rel->rd_rel->relnatts;
> int32 cmp;
> cmp = _bt_compare(state->rel, natts, key, nontarget, upperbound);
> return cmp <= 0;
> }
> It uses scankey, made with _bt_mkscankey() which uses only key
> attributes, but calls _bt_compare with wrong keysz.
> If we wiil use nkeyatts = state->rel->rd_index->relnatts; instead of
> natts, all the checks would be passed successfully.
> Same for invariant_key_greater_than_equal_offset() and
> invariant_key_less_than_equal_nontarget_offset().
> In my view, it's the correct way to fix this problem, because the
> caller is responsible for passing proper arguments to the function.
> Of course I will add a check into bt_compare, but I'd rather make it
> an assertion (see the patch attached).
> I'll add a flag to distinguish regular and truncated tuples, but it
> will not be used in this patch. Please, comment, if I've missed
> something.
> As you've already mentioned, neither high keys, nor tuples on internal
> pages are using "itup->t_tid.ip_posid", so I'll take one bit of it.
> It will definitely require changes in the future works on suffix
> truncation or something like that, but IMHO for now it's enough.
> Do you have any objections or comments?
> [1]

One more version of the patch is attached. I did more testing, and fixed
couple of bugs.
Now, if any indexed column is deleted from table, we perform cascade
deletion of constraint and index.
* Included column deletion leads to the index deletion,
* as well as key columns deletion. It's explained in documentation.
Constraint definition is fixed too.

Also, I added separate regression test for INCLUDING clause, that covers
both indexes and constraints.
I've tested pg_dump, and didn't find any problems. Test script is attached.

It seems to me that the patch is completed.
Except, maybe, grammar check of comments and documentation.

Looking forward to your review.

Anastasia Lubennikova
Postgres Professional:
The Russian Postgres Company

Attachment Content-Type Size
including_columns_8.0.patch text/x-patch 131.7 KB
including_dump_test.sql application/sql 1.3 KB

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