Re: Heap WARM Tuples - Design Draft

On Fri, Aug 5, 2016 at 03:26:15PM -0400, Bruce Momjian wrote:
> > I just don't know how would you do that without delaying/not-doing HOT chain
> > prune. As soon as root and intermediate HOT tuples are gone, all information is
> > lost. You may delay that, but that will defeat the whole purpose. If chains are
> > not pruned in-time, you may not find any free space in the page and end up
> > doing a cold update anyways. But may be I am missing something and Claudio has
> > a different idea.
>
> Yes, pruning would be a problem. :-(
>
> A check only needs to happen when the indexed key changes, right? So we
> are comparing the cost of adding an index key vs. the cost of trying to
> find a matching key/ctid in the index. Seems the later is cheaper, but
> it is not obvious.

Here is an illustration of the issue:

CREATE TABLE test (col1 INTEGER, col2 INTEGER, col3 INTEGER);

-- index first two columns
CREATE INDEX i_test1 ON test (col1);
CREATE INDEX i_test2 ON test (col2);

INSERT INTO test VALUES (1, 7, 20);

-- create a HOT chain
UPDATE test SET col3 = 30;

-- change the HOT chain to a WARM chain, changes i_test2 but not i_test1
UPDATE test SET col2 = 8;

-- we should avoid adding a col2=7 i_test2 index tuple
-- because we already have one; how do we know that?
UPDATE test SET col2 = 7;

-- we should see only one col2=7 i_test2 index tuple
SELECT * FROM test WHERE col2 = 7;
col1 | col2 | col3
------+------+------
1 | 7 | 30

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

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