Thread: Transactions and indexes

On Mon, Jul 19, 2021 at 4:31 PM Chris Cleveland
<ccleveland@dieselpoint.com> wrote:
> I'm confused on how to handle transactions and visibility.

In Postgres, indexes are not considered to be part of the logical
database. They're just data structures that point to TIDs in the
table. To an index, each TID is just another object -- it doesn't
possess any built-in idea about MVCC.

In practice the indexes may be able to surmise certain things about
MVCC and versioning, as an optimization -- but that is all speculative
and relies on cooperation from the table AM side. Also, the
implementation of unique indexes knows more than zero about versions,
since that's just necessary. These two cases may or may not be
considered exceptions to the general rule. I suppose that it's a
matter of perspective.

> So... how do I handle this? Is there some way for me to implement my own storage manager that manages visibility?

This is the responsibility of a table AM, not any one index AM. In
general we assume that each table AM implements something very much
like heapam's VACUUM implementation. Index AMs may also have
opportunistic cleanup of their own, as an optimization (actually this
is what I was referring to).

Theoretically index AMs and table AMs are orthogonal things. How true
that will be in a world with more than one mature table AM remains to
be seen.

-- 
Peter Geoghegan

On Mon, Jul 19, 2021 at 7:20 PM Chris Cleveland
<ccleveland@dieselpoint.com> wrote:
> Thank you. Does this mean I can implement the index AM and return TIDs without having to worry about transactions at
all?

Yes. That's the upside of the design -- it makes it easy to add new
transactional index AMs. Which is one reason why Postgres has so many.

> Also, as far as I can tell, the only way that TIDs are removed from the index is in ambulkdelete(). Is this accurate?

It doesn't have to be the only way, but in practice it can be. Depends
on the index AM. The core code relies on ambulkdelete() to make sure
that all TIDs dead in the table are gone from the index. This allows
VACUUM to finally physically recycle the previously referenced TIDs in
the table structure, without risk of index scans finding the wrong
thing.

> Does that mean that my index will be returning TIDs for deleted items and I don't have to worry about that either?

If you assume that you're using heapam (the standard table AM), then
yes. Otherwise I don't know -- it's ambiguous.

> Don't TIDs get reused? What happens when my index returns an old TID which is now pointing to a new record?

This can't happen because, as I said, the table cannot recycle
TIDs/line pointers until it's known that this cannot happen (because
VACUUM already cleaned out all the garbage index tuples).

> This is going to make it really hard to implement Top X queries of the type you get from a search engine. A search
enginewill normally maintain an internal buffer (usually a priority queue) of a fixed size, X, and add tuples to it
alongwith their relevance score. The buffer only remembers the Top X tuples with the highest score. In this way the
searchengine can iterate over millions of entries and retain only the best ones without having an unbounded buffer. For
thisto work, though, you need to know how many tuples to keep in the buffer in advance. If my index can't know, in
advance,which TIDs are invisible or deleted, then it can't keep them out of the buffer, and this whole scheme fails. 
>
> This is not going to work unless the system gives the index a clear picture of transactions, visibility, and deletes
asthey happen. Is this information available? 

Are you implementing a new index AM or a new table AM? Discarding data
based on something like a relevance score doesn't seem like something
that either API provides for. Indexes in Postgres can be lossy, but
that in itself doesn't change the result of queries.

--
Peter Geoghegan