Previously, TidStoreIterateNext() would expand the set of offsets for
each block into an internal buffer that it overwrote each time. In
order to be able to collect the offsets for multiple blocks before
working with them, change the contract. Now, the offsets are obtained
by a separate call to TidStoreGetBlockOffsets(), which can be called at
a later time. TidStoreIteratorResult objects are safe to copy and store
in a queue.
Reviewed-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/CAAKRu_bbkmwAzSBgnezancgJeXrQZXy4G4kBTd+5=cr86H5yew@mail.gmail.com
Previously, the behavior of TidStoreCreate() was inconsistent between
local and shared TidStore instances in terms of memory limitation. For
local TidStore, a memory context was created with initial and maximum
memory block sizes, as well as a minimum memory context size, based on
the specified max_bytes values. However, for shared TidStore, the
provided DSA area was used for TID storage. Although commit bb952c8c8b
allowed specifying the initial and maximum DSA segment sizes, callers
would have needed to clamp their own limits, which was not consistent
and user-friendly.
With this commit, when creating a shared TidStore, a dedicated DSA
area is created for TID storage instead of using a provided DSA
area. The initial and maximum DSA segment sizes are chosen based on
the specified max_bytes. Other processes can attach to the shared
TidStore using the handle of the created DSA returned by the new
TidStoreGetDSA() function and the DSA pointer returned by
TidStoreGetHandle(). The created DSA has the same lifetime as the
shared TidStore and is deleted when all processes detach from it.
To improve clarity, the TidStoreCreate() function has been divided
into two separate functions: TidStoreCreateLocal() and
TidStoreCreateShared().
Reviewed-by: John Naylor
Discussion: https://postgr.es/m/CAD21AoAyc1j%3DBCdUqZfk6qbdjZ68UgRx1Gkpk0oah4K7S0Ri9g%40mail.gmail.com
TIDStore is a data structure designed to efficiently store large sets
of TIDs. For TID storage, it employs a radix tree, where the key is
a block number, and the value is a bitmap representing offset
numbers. The TIDStore can be created on a DSA area and used by
multiple backend processes simultaneously.
There are potential future users such as tidbitmap.c, though it's very
likely the interface will need to evolve as we come to understand the
needs of different kinds of users. For example, we can support
updating the offset bitmap of existing values.
Currently, the TIDStore is not used for anything yet, aside from the
test code. But an upcoming patch will use it.
This includes a unit test module, in src/test/modules/test_tidstore.
Co-authored-by: John Naylor
Discussion: https://postgr.es/m/CAD21AoAfOZvmfR0j8VmZorZjL7RhTiQdVttNuC4W-Shdc2a-AA%40mail.gmail.com
