Fix 'Unknown' placeholder in execution plan output for subplan filter cost model

Environment

• Version: 5.7.25 OceanBase seekdb (r) (Built 1-e1ac5a57bf17078ca46ec9add574edeffac7c3f9 Apr 7 2026)
• Is this an RD-provided temporary version?: No
• Reproducibility: Stable reproduction

Description

When running the cost_model.subplan_filter_mysql test case, the execution plan output contains an Unknown placeholder where the actual correlated subquery condition should be displayed.

Steps to Reproduce

1. Connect using: mysql -uroot -h -P 2881
2. Run the following SQL statements:

drop table if exists t1_subplan_filter;
create table t1_subplan_filter(
pk int primary key,
c1 int,
c2 number,
c3 varchar(10),
c4 json,
c5 text);
create index i1 on t1_subplan_filter(c2);
call dbms_stats.set_table_stats(
NULL,
't1_subplan_filter',
numrows => 1000000,
avgrlen => 100,
nummicroblks => 6000
);
explain select (select /*+no_unnest*/ count(*) from t1_subplan_filter) as ref from t1_subplan_filter;

3. Observe the execution plan output. The issue is visible in the attached image.

Environment Information

Please provide the connection strings for the problematic tenant and the sys tenant, as well as deployment information.
Has the scene been preserved?

Log Information

No logs provided.

Other Notes

This issue was reported from an external business perspective. Please select the appropriate "Work Item Type" on the right based on the actual situation:

• Customer Service Feedback: Refers specifically to issues before production deployment or during POC, where a product bug is confirmed to require a fix and release.
• Online Issue: A problem that has occurred after production deployment, confirmed as a product bug but not classified as a fault.
• Online Fault: A problem that has occurred after production deployment, confirmed as a product bug, and has caused business errors or public impact on the customer side (the definition of a fault is at the discretion of the delivery service team).

Start with the cost_model.subplan_filter_mysql test case and reproduce the EXPLAIN output using the SQL in the issue. Trace where the correlated subquery condition is rendered in the execution plan, then verify that the output shows that condition instead of "Unknown".