Re: Parallel CREATE INDEX for GIN indexes

On 5/13/24 10:19, Andy Fan wrote:
>
> Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com> writes:
>
>> ...
>>
>> I don't understand the question. The blocks are distributed to workers
>> by the parallel table scan, and it certainly does not do that block by
>> block. But even it it did, that's not a problem for this code.
>
> OK, I get ParallelBlockTableScanWorkerData.phsw_chunk_size is designed
> for this.
>
>> The problem is that if the scan wraps around, then one of the TID lists
>> for a given worker will have the min TID and max TID, so it will overlap
>> with every other TID list for the same key in that worker. And when the
>> worker does the merging, this list will force a "full" merge sort for
>> all TID lists (for that key), which is very expensive.
>
> OK.
>
> Thanks for all the answers, they are pretty instructive!
>

Thanks for the questions, it forces me to articulate the arguments more
clearly. I guess it'd be good to put some of this into a README or at
least a comment at the beginning of gininsert.c or somewhere close.

regards

--
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company