Re: Lossy Index Tuple Enhancement (LITE)

On Thu, Aug 4, 2016 at 1:58 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
> On 3 August 2016 at 20:37, Claudio Freire <klaussfreire@gmail.com> wrote:
>> On Wed, Aug 3, 2016 at 4:20 AM, Simon Riggs <simon@2ndquadrant.com> wrote:
>>> == IndexScan ==
>>>
>>> Note that the executor code for IndexScan appears identical between
>>> the two optimizations. The difference between duplicate and range LITE
>>> tuples is needed only at INSERT time (or UPDATE indexed column to a
>>> new value).
>>>
>>> When we do an IndexScan if we see a LITE tuple we do a scan of the
>>> linepointer ranges covered by this index tuple (which might eventually
>>> go to a full block scan). For example with bit1 set we would scan 16
>>> linepointers (on an 8192 byte block) and with 2 bits set we would scan
>>> 32 linepointers etc.., not necessarily consecutive ranges. So the
>>> IndexScan can return potentially many heap rows per index entry, which
>>> need to be re-checked and may also need to be sorted prior to
>>> returning them. If no rows are returned we can kill the index pointer,
>>> just as we do now for btrees, so they will be removed eventually from
>>> the index without the need for VACUUM. An BitmapIndexScan that sees a
>>> lossy pointer can also use the lossy TID concept, so we can have
>>> partially lossy bitmaps.
>>
>> Wouldn't this risk scanning rows more than once unless it's part of a
>> bitmap scan?
>
> It's always the job of the index insert logic to ensure a valid index exists.
>
> I'm not sure what you see that changes that here. Say more?

I just explained it further in the WARM thread, which is actually the
idea I was arriving to.