Re: group locking: incomplete patch, just for discussion

On Wed, Oct 15, 2014 at 12:13 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> What is "timely service lock requests"? You got the lock before firing
> off the background workers, you hold it till they're done.

If you want to run any non-trivial (read: useful) code in the
background workers, it rapidly gets very hard to predict which
relation locks they might need, and infeasible to hold them for the
lifetime of the computation. For example, suppose we restrict
background workers to running only operators found in btree opclasses.
That's a far more draconian restriction than we'd like to have, but
perhaps liveable in a pinch. But bttextcmp() uses
PG_GETARG_TEXT_PP(), which means it may (or may not) need to lock the
TOAST table; and it can indirectly call lookup_collation_cache() which
does SearchSysCache1(COLLOID, ...) which may result in scanning
pg_collation. And enum_cmp_internal() will do
SearchSysCache1(ENUMOID, ...) which may result in scanning pg_enum.

There's obviously a balance to be struck, here. It will never be safe
to run just anything in a background worker, and we'll go crazy if we
try to make that work. On the other hand, if there's essentially no
code that can run in a background worker without extensive
modification, we might as well not bother implementing parallelism in
the first place. I admit that getting group locking is far from
simple, but I also don't believe it's as complicated as previous
projects like SSI, logical decoding, or LATERAL.

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