Re: enhance the efficiency of migrating particularly large tables
On Fri, 3 May 2024 at 09:33, David Zhang wrote: > Is there a simple way to get the min of ctid faster than using min(), but > similar to get the max of ctid using pg_relation_size? The equivalent approximation is always '(0,1)'. David
Re: enhance the efficiency of migrating particularly large tables
Thanks a lot David Rowley for your suggestion in details.
On 2024-04-08 3:23 p.m., David Rowley wrote:
On Tue, 9 Apr 2024 at 09:52, David Zhang wrote:
Finding the exact ctid seems overkill for what you need. Why you
could just find the maximum block with:
N = pg_relation_size('name_of_your_table'::regclass) /
current_Setting('block_size')::int;
and do WHERE ctid < '(N,1)';
We experienced this approach using pg_relation_size and tried to compare
the performance. Below are some simple timing results for 100 million
records in a table:
Using system function max():
SELECT max(ctid) from t;
Time: 2126.680 ms (00:02.127)
Using pg_relation_size and where condition:
SELECT pg_relation_size('t'::regclass) / current_setting('block_size')::int;
Time: 0.561 ms
Using the experimental function introduced in previous patch:
SELECT ctid from get_ctid('t', 1);
Time: 0.452 ms
Delete about 1/3 records from the end of the table:
SELECT max(ctid) from t;
Time: 1552.975 ms (00:01.553)
SELECT pg_relation_size('t'::regclass) / current_setting('block_size')::int;
Time: 0.533 m
But before vacuum, pg_relation_size always return the same value as
before and this relation_size may not be so accurate.
SELECT ctid from get_ctid('t', 1);
Time: 251.105 m
After vacuum:
SELECT ctid from get_ctid('t', 1);
Time: 0.478 ms
Below are the comparison between system function min() and the
experimental function:
SELECT min(ctid) from t;
Time: 1932.554 ms (00:01.933)
SELECT ctid from get_ctid('t', 0);
Time: 0.478 ms
After deleted about 1/3 records from the beginning of the table:
SELECT min(ctid) from t;
Time: 1305.799 ms (00:01.306)
SELECT ctid from get_ctid('t', 0);
Time: 244.336 ms
After vacuum:
SELECT ctid from get_ctid('t', 0);
Time: 0.468 ms
If we wanted to optimise this in PostgreSQL, the way to do it would
be, around set_plain_rel_pathlist(), check if the relation's ctid is a
required PathKey by the same means as create_index_paths() does, then
if found, create another seqscan path without synchronize_seqscans *
and tag that with the ctid PathKey sending the scan direction
according to the PathKey direction. nulls_first does not matter since
ctid cannot be NULL.
Min(ctid) query should be able to make use of this as the planner
should rewrite those to subqueries with a ORDER BY ctid LIMIT 1.
Is there a simple way to get the min of ctid faster than using min(),
but similar to get the max of ctid using pg_relation_size?
Thank you,
David Zhang
Re: enhance the efficiency of migrating particularly large tables
On Tue, 9 Apr 2024 at 11:02, Tom Lane wrote: > > David Rowley writes: > > Unsure if such a feature is worthwhile. I think maybe not for just > > min(ctid)/max(ctid). However, there could be other reasons, such as > > the transform OR to UNION stuff that Tom worked on a few years ago. > > That needed to eliminate duplicate rows that matched both OR branches > > and that was done using ctid. > > I'm kind of allergic to adding features that fundamentally depend on > ctid, seeing that there's so much activity around non-heap table AMs > that may not have any such concept, or may have a row ID that looks > totally different. (That's one reason why I stopped working on that > OR-to-UNION patch.) I understand that point of view, however, I think if we were to maintain it as a policy that we'd likely miss out on various optimisations that future AMs could provide. When I pushed TID Range Scans a few years ago, I added "amflags" and we have AMFLAG_HAS_TID_RANGE so the planner can check the AM supports that before adding the Path. Anyway, I'm not saying let's do the non-sync scan SeqScanPath thing, I'm just saying that blocking optimisations as some future AM might not support it might mean we're missing out on some great speedups. David
Re: enhance the efficiency of migrating particularly large tables
David Rowley writes: > Unsure if such a feature is worthwhile. I think maybe not for just > min(ctid)/max(ctid). However, there could be other reasons, such as > the transform OR to UNION stuff that Tom worked on a few years ago. > That needed to eliminate duplicate rows that matched both OR branches > and that was done using ctid. I'm kind of allergic to adding features that fundamentally depend on ctid, seeing that there's so much activity around non-heap table AMs that may not have any such concept, or may have a row ID that looks totally different. (That's one reason why I stopped working on that OR-to-UNION patch.) regards, tom lane
Re: enhance the efficiency of migrating particularly large tables
On Tue, 9 Apr 2024 at 09:52, David Zhang wrote:
> However, when executing SELECT min(ctid) and max(ctid), it performs a
> Seq Scan, which can be slow for a large table. Is there a way to
> retrieve the minimum and maximum ctid other than using the system
> functions min() and max()?
Finding the exact ctid seems overkill for what you need. Why you
could just find the maximum block with:
N = pg_relation_size('name_of_your_table'::regclass) /
current_Setting('block_size')::int;
and do WHERE ctid < '(N,1)';
If we wanted to optimise this in PostgreSQL, the way to do it would
be, around set_plain_rel_pathlist(), check if the relation's ctid is a
required PathKey by the same means as create_index_paths() does, then
if found, create another seqscan path without synchronize_seqscans *
and tag that with the ctid PathKey sending the scan direction
according to the PathKey direction. nulls_first does not matter since
ctid cannot be NULL.
Min(ctid) query should be able to make use of this as the planner
should rewrite those to subqueries with a ORDER BY ctid LIMIT 1.
* We'd need to invent an actual Path type for SeqScanPath as I see
create_seqscan_path() just uses the base struct Path.
synchronize_seqscans would have to become a property of that new Path
type and it would need to be carried forward into the plan and looked
at in the executor so that we always start a scan at the first or last
block.
Unsure if such a feature is worthwhile. I think maybe not for just
min(ctid)/max(ctid). However, there could be other reasons, such as
the transform OR to UNION stuff that Tom worked on a few years ago.
That needed to eliminate duplicate rows that matched both OR branches
and that was done using ctid.
David
