Re: Tid scan improvements

On Fri, 15 Mar 2019 at 18:42, Edmund Horner <ejrh00(at)gmail(dot)com> wrote:
> I've had to adapt it to use the table scan API. I've got it compiling
> and passing tests, but I'm uneasy about some things that still use the
> heapam API.
>
> 1. I call heap_setscanlimits as I'm not sure there is a tableam equivalent.
> 2. I'm not sure whether non-heap tableam implementations can also be
> supported by my TID Range Scan: we need to be able to set the scan
> limits. There may not be any other implementations yet, but when
> there are, how do we stop the planner using a TID Range Scan for
> non-heap relations?
> 3. When fetching tuples, I see that nodeSeqscan.c uses
> table_scan_getnextslot, which saves dealing with HeapTuples. But
> nodeTidrangescan wants to do some checking of the block and offset
> before returning the slot. So I have it using heap_getnext and
> ExecStoreBufferHeapTuple. Apart from being heapam-specific, it's just
> not as clean as the new API calls.
>
> Ideally, we can get to to support general tableam implementations
> rather than using heapam-specific calls. Any advice on how to do
> this?

The commit message in 8586bf7ed mentions:

> Subsequent commits will incrementally abstract table access
> functionality to be routed through table access methods. That change
> is too large to be reviewed & committed at once, so it'll be done
> incrementally.

and looking at [1] I see patch 0004 introduces some changes in
nodeTidscan.c to call a new tableam API function named
heapam_fetch_row_version. I see this function does have a ItemPointer
argument, so I guess we must be keeping those as unique row
identifiers in the API.

Patch 0001 does change the signature of heap_setscanlimits() (appears
to be committed already), and then in 0010 the only code that calls
heap_setscanlimits() (IndexBuildHeapRangeScan()) is moved and renamed
to heapam_index_build_range_scan() and set to be called via the
index_build_range_scan TableAmRoutine method. So it looks like out of
that patch series nothing is there to allow you to access
heap_setscanlimits() directly via the TableAmRoutine API, so perhaps
for this to work heap_setscanlimits will need to be interfaced,
however, I'm unsure if that'll violate any assumptions that Andres
wants to keep out of the API... Andres?

[1] https://www.postgresql.org/message-id/20190311193746.hhv4e4e62nxtq3k6@alap3.anarazel.de

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