Re: enhance the efficiency of migrating particularly large tables

Thanks a lot David Rowley for your suggestion in details.

On Tue, 9 Apr 2024 at 09:52, David Zhang <david.zhang@highgo.ca> 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