On Fri, Aug 5, 2016 at 12:44 AM, Bruce Momjian <bruce@momjian.us> wrote:
>> UPDATE t SET dat = dat + 1, id = 3, someid = 3 where id = 2;
>
> This is ends the WARM chain, and creates new index entries because all
> indexes are changed.
>
>> UPDATE t SET dat = dat + 1, id = 1, someid = 2 where id = 3;
>
> This does the same thing.
>
>> SELECT * FROM t WHERE someid = 2;
>
> This uses the 'someid' index. The index contains three entries:
>
> 1. {someid=2} pointing to first WARM chain
> 2. {someid=3} pointing to single tuple (no HOT chain)
> 3. {someid=2} pointing to single tuple (no HOT chain)
>
> The scan of #1 returns no visible rows. #2 doesn't match the value in
> the WHERE clause, so we don't even check the heap. The scan of #3
> returns one row.
>
> Remember, we don't scan past the end of the HOT chain, which is what we
> do now.
Ok, I botched the example.
I wanted the other updates to all be WARM updates, so imagine there's
another index that is unchanged (say, a someid2 not updated ever).
