Re: [PoC] Improve dead tuple storage for lazy vacuum

On Mon, Nov 27, 2023 at 1:45 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Sat, Oct 28, 2023 at 5:56 PM John Naylor <johncnaylorls(at)gmail(dot)com> wrote:

> bool
> RT_SET(RT_RADIX_TREE *tree, uint64 key, RT_VALUE_TYPE *value_p);
> or for variable-length value support,
> RT_SET(RT_RADIX_TREE *tree, uint64 key, RT_VALUE_TYPE *value_p, size_t sz);
>
> If an entry already exists, update its value to 'value_p' and return
> true. Otherwise set the value and return false.

> RT_VALUE_TYPE
> RT_INSERT(RT_RADIX_TREE *tree, uint64 key, size_t sz, bool *found);
>
> If the entry already exists, replace the value with a new empty value
> with sz bytes and set "found" to true. Otherwise, insert an empty
> value, return its pointer, and set "found" to false.
>
> We probably will find a better name but I use RT_INSERT() for
> discussion. RT_INSERT() returns an empty slot regardless of existing
> values. It can be used to insert a new value or to replace the value
> with a larger value.

Looking at TidStoreSetBlockOffsets again (in particular how it works
with RT_GET), and thinking about issues we've discussed, I think
RT_SET is sufficient for vacuum. Here's how it could work:

TidStoreSetBlockOffsets could have a stack variable that's "almost
always" large enough. When not, it can allocate in its own context. It
sets the necessary bits there. Then, it passes the pointer to RT_SET
with the number of bytes to copy. That seems very simple.

At some future time, we can add a new function with the complex
business about getting the current value to modify it, with the
re-alloc'ing that it might require.

In other words, from both an API perspective and a performance
perspective, it makes sense for tid store to have a simple "set"
interface for vacuum that can be optimized for its characteristics
(insert only, ordered offsets). And also a more complex one for bitmap
scan (setting/unsetting bits of existing values, in any order). They
can share the same iteration interface, key types, and value types.

What do you think, Masahiko?