Re: Parallel bitmap index scan - Mailing list pgsql-hackers

On Sun, Jul 26, 2020 at 6:43 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:

>

> I would like to propose a patch for enabling the parallelism for the

> bitmap index scan path.

>

> Background:

> Currently, we support only a parallel bitmap heap scan path.  Therein,

> the underlying bitmap index scan is done by a single worker called the

> leader.  The leader creates a bitmap in shared memory and once the

> bitmap is ready it creates a shared iterator and after that, all the

> workers process the shared iterator and scan the heap in parallel.

> While analyzing the TPCH plan we have observed that some of the

> queries are spending significant time in preparing the bitmap.  So the

> idea of this patch is to use the parallel index scan for preparing the

> underlying bitmap in parallel.

>

> Design:

> If underlying index AM supports the parallel path (currently only

> BTREE support it), then we will create a parallel bitmap heap scan

> path on top of the parallel bitmap index scan path.  So the idea of

> this patch is that each worker will do the parallel index scan and

> generate their part of the bitmap.  And, we will create a barrier so

> that we can not start preparing the shared iterator until all the

> worker is ready with their bitmap.  The first worker, which is ready

> with the bitmap will keep a copy of its TBM and the page table in the

> shared memory.  And, all the subsequent workers will merge their TBM

> with the shared TBM.  Once all the TBM are merged we will get one

> common shared TBM and after that stage, the worker can continue.  The

> remaining part is the same,  basically, again one worker will scan the

> shared TBM and prepare the shared iterator and once it is ready all

> the workers will jointly scan the heap in parallel using shared

> iterator.

>



Though I have not looked at the patch or code for the existing

parallel bitmap heap scan, one point keeps bugging in my mind. I may

be utterly wrong or my question may be so silly, anyways I would like

to ask here:



From the above design: each parallel worker creates partial bitmaps

for the index data that they looked at. Why should they merge these

bitmaps to a single bitmap in shared memory? Why can't each parallel

worker do a bitmap heap scan using the partial bitmaps they built

during it's bitmap index scan and emit qualified tuples/rows so that

the gather node can collect them? There may not be even lock

contention as bitmap heap scan takes read locks for the heap

pages/tuples.