Alvaro,
I suppose there must be reasons not to do this, but have you considered using the "slack" space (empty space) in an ordinary table "heap" page to store share-locks on the tuples in that page? (If not enough space is available then you would still need to use the spilled-to-disk btree structure.)
Maybe there could also be something that you put in the disk page that means "transaction X has all the tuples in this page share-locked," since I imagine it will usually be the case that if very many of them are locked, then they are all locked.
With this method, checking for a lock would be slightly more complicated since you would need to check the disk page in which the tuple resides first, and the spill-to-disk structure afterwards.
It seems that if this would work, you would be able to share-lock every tuple in a table (or a large fraction of them) without a lot of extra IO provided that the pages were not 100 % full. (Which is almost never the case in postgresql anyway.)
Paul Tillotson
Alvaro Herrera wrote:
On Thu, Mar 31, 2005 at 06:54:31PM -0600, Guy Rouillier wrote:
Alvaro Herrera wrote:
Now this can't be applied right away because it's easy to run "out ofWhile not always true, in many cases the cardinality of the referenced
memory" (shared memory for the lock table). Say, a delete or update
that touches 10000 tuples does not work. I'm currently working on a
proposal to allow the lock table to spill to disk ...
(parent) table is small compared to that of the referencing (child)
table. Does locking require a separate lock record for each tuple in
the child table, or just one for each tuple in the parent table with a
reference count?
Just one. (LOCALLOCK, which is private to each backend, stores how many times we hold a lock.)
I just realized we not only need to be able to spill LOCK struct to disk, but also PROCLOCK ... am I right?
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