On 11/03/2016 03:59 PM, Robert Haas wrote:
On Wed, Nov 2, 2016 at 12:49 PM, Tomas Vondra
<tomas.von...@2ndquadrant.com> wrote:
On 11/01/2016 08:32 PM, Robert Haas wrote:
On Tue, Nov 1, 2016 at 10:58 AM, Tomas Vondra
<tomas.von...@2ndquadrant.com> wrote:
Damn! You're right of course. Who'd guess I need more coffee this early?
Attached is a fix replacing the flag with an array of flags, indexed by
ParallelMasterBackendId. Hopefully that makes it work with multiple
concurrent parallel queries ... still, I'm not sure this is the right
solution.
I feel like it isn't. I feel like this ought to go in the DSM for
that parallel query, not the main shared memory segment, but I'm not
sure how to accomplish that offhand. Also, if we do solve it this
way, surely we don't need the locking. The flag's only set before any
workers have started and never changes thereafter.
I'm not sure what you mean by "DSM for that parallel query" - I thought the
segments are created for Gather nodes, no? Or is there a DSM for the whole
query that we could use?
Sure, the Gather node creates it. There's generally only one per
query, though, and that's how most information is communicated from
leader to workers.
Ah, right. I haven't realized there's just a single Gather per query.
>>
Another thing is that maybe we don't really want to give extensions access
to any of those segments - my impression was those segments are considered
internal (is there RequestAddinShmemSpace for them?). And hacking something
just for auto_explain seems a big ugly.
Yeah. I thought that there wouldn't be any reason for third-party
code to need to get into these segments, but maybe that was
short-sighted of me. If we fix this without that, then we've got to
force pg_stat_statements to be loaded through
shared_preload_libarries, as you mentioned, and that doesn't seem nice
either.
Well, I was talking about auto_explain, and pg_stat_statements already
has to be loaded through shared_preload_libraries. I'm not sure how many
other extensions would need similar hack (I don't see any in contrib,
but there may be external ones).
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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