feat(subscription): support cusor order

[xxhZs]

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What's changed and what's your intention?

#18208

In this pr, we'll make sure that the results returned by cursor are sorted by pk, making sure that we return the results in only one order, this is in order to later use pk+rw_timestamp+op to implement the cursor Exactly Once

And support explain fetch n from subscription cursor will return the batch plan of subscription cursor

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[hzxa21]

Discussed offline: we can support explain fetch ... <cursor> in this PR so that we can understand more about the batch query plan.

[hzxa21]

remove

[hzxa21]

Please include the epoch range and version id in the result as well.

[hzxa21]

It seems that the ordering doesn't work. Example:

dev=> create table a(v int, pk int primary key);
CREATE_TABLE
dev=> create subscription b from a with (retention = '1d');
CREATE_SUBSCRIPTION
dev=> declare cursor subscription cursor for b;
DECLARE_CURSOR
dev=> insert into a values(120, 25),(119, 23);
INSERT 0 2
dev=> flush;
FLUSH
dev=> fetch 2 from cursor;
  v  | pk |   op   | rw_timestamp  
-----+----+--------+---------------
 120 | 25 | Insert | 1729493767419
 119 | 23 | Insert | 1729493767419
(2 rows)

dev=> explain fetch 2 from cursor;
                      QUERY PLAN                      
------------------------------------------------------
 BatchExchange { order: [a.pk ASC], dist: Single }
 └─BatchLogSeqScan { table: a, columns: [v, pk, op] }
(2 rows)

[hzxa21]

Like the normal batch iterator, can we have a ordered option to determine whether merge_sort is needed?

See

risingwave/src/batch/src/executor/row_seq_scan.rs

Line 238 in d8199b5

let ordered = seq_scan_node.ordered;

and

risingwave/src/storage/src/table/batch_table/storage_table.rs

Line 539 in d8199b5

_ if !ordered => {

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[hzxa21]

Rest LGTM!

[hzxa21]

Let's make this consistent with BatchSeqScan:

ordered: !self.order().is_any()