Horton

Re: Pluggable Storage - Andres's take

Haribabu Kommi <kommi.haribabu@gmail.com>

View thread View on postgr.es
From: Haribabu Kommi <kommi.haribabu@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>, Alexander Korotkov <a.korotkov@postgrespro.ru>, Pg Hackers <pgsql-hackers@postgresql.org>
Date: 2018-09-21T07:40:10Z
Lists: pgsql-hackers 
On Fri, Sep 21, 2018 at 5:05 PM Andres Freund <andres@anarazel.de> wrote:

> Hi,
>
> On 2018-09-21 16:57:43 +1000, Haribabu Kommi wrote:
>
> > For example, in the sequential scan, the heap returns the slot with
> > the tuple or with value array of all the columns and then the data gets
> > filtered and later removed the unnecessary columns with projection.
> > This works fine for the row based storage. For columnar storage, if
> > the storage knows that upper layers needs only particular columns,
> > then they can directly return the specified columns and there is no
> > need of projection step. This will help the columnar storage also
> > to return proper columns in a faster way.
>
> I think this is an important feature, but I feel fairly strongly that we
> should only tackle it in a second version. This patchset is already
> pretty darn large.  It's imo not just helpful for columnar, but even for
> heap - we e.g. spend a lot of time deforming columns that are never
> accessed. That's particularly harmful when the leading columns are all
> NOT NULL and fixed width, but even if not, it's painful.
>

OK. Thanks for your opinion.
Then I will first try to cleanup the open items of the existing patch.


> Is it good to pass the plan to the storage, so that they can find out
> > the columns that needs to be returned?
>
> I don't think that's the right approach - this should be a level *below*
> plan nodes, not reference them. I suspect we're going to have to have a
> new table_scan_set_columnlist() option or such.
>

The table_scan_set_columnlist() API can be a good solution to share
the columns that are expected.



> > And also if the projection can handle in the storage itself for some
> > scenarios, need to be informed the callers that there is no need to
> > perform the projection extra.
>
> I don't think that should be done in the storage layer - that's probably
> better done introducing custom scan nodes and such.  This has costing
> implications etc, so this needs to happen *before* planning is finished.
>

Sorry, my explanation was wrong, Assuming a scenario where the target list
contains only the plain columns of a table and these columns are already
passed
to storage using the above proposed new API and their of one to one mapping.
Based on the above info, deciding whether the projection is required or not
is good.

Regards,
Haribabu Kommi
Fujitsu Australia

Commits

  1. Add default_table_access_method to postgresql.conf.sample.

  2. tableam: Avoid relying on relation size to determine validity of tids.

  3. tableam: Don't assume that every AM uses md.c style storage.

  4. Allow pg_class xid & multixid horizons to not be set.

  5. Fix slot type issue for fuzzy distance index scan over out-of-core table AM.

  6. tableam: comment and formatting fixes.

  7. table: docs: fix typos and grammar.

  8. tableam: basic documentation.

  9. Only allow heap in a number of contrib modules.

  10. tableam: Add table_finish_bulk_insert().

  11. tableam: sample scan.

  12. tableam: bitmap table scan.

  13. tableam: Move heap specific logic from estimate_rel_size below tableam.

  14. tableam: VACUUM and ANALYZE support.

  15. tableam: relation creation, VACUUM FULL/CLUSTER, SET TABLESPACE.

  16. tableam: Support for an index build's initial table scan(s).

  17. tableam: Add table_get_latest_tid, to wrap heap_get_latest_tid.

  18. tableam: Add helper for indexes to check if a corresponding table tuples exist.

  19. tableam: Add and use table_fetch_row_version().

  20. tableam: Use in CREATE TABLE AS and CREATE MATERIALIZED VIEW.

  21. Ensure sufficient alignment for ParallelTableScanDescData in BTShared.

  22. Don't reuse slots between root and partition in ON CONFLICT ... UPDATE.

  23. Use a virtual rather than a heap slot in two places where that suffices.

  24. Store tuples for EvalPlanQual in slots, rather than as HeapTuples.

  25. Use slots in trigger infrastructure, except for the actual invocation.

  26. Store table oid and tuple's tid in tuple slots directly.

  27. Allow to use HeapTupleData embedded in [Buffer]HeapTupleTableSlot.

  28. Add ExecStorePinnedBufferHeapTuple.

  29. Add ArchiveOpts to pass options to ArchiveEntry

  30. Rename RelationData.rd_amroutine to rd_indam.

  31. Rephrase references to "time qualification".

  32. Move remaining code from tqual.[ch] to heapam.h / heapam_visibility.c.

  33. Move generic snapshot related code from tqual.h to snapmgr.h.

  34. Remove superfluous tqual.h includes.

  35. Don't duplicate parallel seqscan shmem sizing logic in nbtree.

  36. Move vacuumlazy.c into access/heap.

  37. Support parallel bitmap heap scans.