Re: Parallel Seq Scan

On Fri, Oct 16, 2015 at 2:29 AM, Amit Kapila <amit.kapila16@gmail.com> wrote:
>> Yeah, but I think the scenario is legitimate.  When a query gets run
>> from within PL/pgsql, parallelism is an option, at least as we have
>> the code today.  So if a Gather were present, and the query used a
>> parameter, then you could have this issue.  For example:
>>
>> SELECT * FROM bigtable WHERE unindexed_column = some_plpgsql_variable;
>>
>
> I don't think for such statements the control flow will set up an unshared
> param list.  I have tried couple of such statements [1] and found that
> always such parameters are set up by setup_param_list().  I think there
> are only two possibilities which could lead to setting up of unshared
> params:
>
> 1. Usage of cursors - This is already prohibited for parallel-mode.
> 2. Usage of read-write-param - This only happens for expressions like
> x := array_append(x, foo) (Refer exec_check_rw_parameter()).  Read-write
> params are not used for SQL statements. So this also won't be used for
> parallel-mode
>
> There is a chance that I might be missing some case where unshared
> params will be required for parallel-mode (as of today), but if not then
> I think we can live without current changes.

*shrug*

The gather-test stuff isn't failing for no reason.  Either PL/pgsql
shouldn't be passing CURSOR_OPT_PARALLEL_OK, or having a parallel plan
get generated there should work.  There's not a third option.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company