sql/schemachanger: dropping a column after add in the same transaction fails

[spilchen]

Describe the problem

The declarative schema changer fails to build a proper plan when adding and dropping columns in separate statements within the same transaction.

To Reproduce

CREATE TABLE t1 (c1 INT NOT NULL PRIMARY KEY, c2 TEXT NOT NULL, drop DATE, FAMILY F1 (c1, c2, drop));
set use_declarative_schema_changer = 'unsafe_always';
begin;
ALTER TABLE t1 ADD COLUMN c3 TEXT DEFAULT '<default>';
ALTER TABLE t1 DROP COLUMN drop;
commit;

The drop column statement fails with this error:

ERROR: error executing StatementPhase stage 1 of 1 with 3 MutationType ops: relation "t1" (112): index "crdb_internal_index_2_name_placeholder" contains stored column "" with unknown ID 0
DETAIL: • Schema change plan for ALTER TABLE ‹movr›.public.‹t1› DROP COLUMN ‹drop›; following ALTER TABLE ‹movr›.public.‹t1› ADD COLUMN ‹c3› STRING DEFAULT ‹'<default>'›;

Observations:

• Reversing the order (DROP first, then ADD) or combining both actions in a single statement avoids the issue.

Environment:

• CockroachDB version: master branch 5d084bd

Jira issue: CRDB-45021

[blathers-crl]

Hi @spilchen, please add branch-* labels to identify which branch(es) this C-bug affects.

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