sql/schemachanger: copy ALTER TABLE .. SET TYPE validation checks from legacy to DSC

[spilchen]

This duplicates various validation checks from the legacy schema changer's ALTER TABLE .. SET TYPE command for the declarative schema changer, which uses states from various schema changer elements.

Additionally, it introduces more tests for validation-only type conversions. These conversions don't require data to be rewritten; instead, they validate existing rows against the new type. For example, changing from CHAR(20) to CHAR(10) requires ensuring each row is 10 characters or fewer.

The actual implementation of validation-only type conversions in the DSC will be handled in a subsequent PR.

Epic: CRDB-25314
Informs: #127516
Release note: None

[cockroach-teamcity]

This change is 

[fqazi]

Great work, :

Just some question, but no issues with this work.

Reviewed 7 of 7 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @spilchen)

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pkg/sql/alter_column_type.go line 195 at r1 (raw file):

		}
	}
	for _, fk := range tableDesc.InboundForeignKeys() {

Did we have a bug before? Or was this enforced else where?

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pkg/sql/logictest/testdata/logic_test/alter_column_type line 1051 at r1 (raw file):

# rows will not fit in the new type. The data of existing rows gets truncated,
# which isn't correct. This will get addressed when we add the validation
# only logic in #127516.

Do we know what the behaviour of Postgres for similar scenarios? Validation will definitely make this safer and remove risk of data loss.

[spilchen]

TFTR @fqazi. I responded to your comments.

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @fqazi)

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pkg/sql/alter_column_type.go line 195 at r1 (raw file):

Previously, fqazi (Faizan Qazi) wrote…

Did we have a bug before? Or was this enforced else where?

This was a bug before. The alter_column_type logic test tried to verify this, but it was reporting the wrong error message. It was exposed when I converted this check to the DSC. This update to the test corrects that problem:

statement ok
CREATE TABLE t19 (y INT NOT NULL REFERENCES t18 (x), INDEX(y));

- statement error pq: unimplemented: ALTER COLUMN TYPE requiring rewrite of on-disk data is currently not supported for columns that are part of an index
+ statement error pq: unimplemented: ALTER COLUMN TYPE for a column that has a constraint is currently not supported
ALTER TABLE t18 ALTER COLUMN x TYPE STRING

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pkg/sql/logictest/testdata/logic_test/alter_column_type line 1051 at r1 (raw file):

Previously, fqazi (Faizan Qazi) wrote…

Do we know what the behaviour of Postgres for similar scenarios? Validation will definitely make this safer and remove risk of data loss.

Yes, postgres will validate that the existing data fits and fails when it doesn't.

postgres=# create table k1 (c1 text);
CREATE TABLE
postgres=# insert into k1 values ('looonnnnggggg string');
INSERT 0 1
postgres=# alter table k1 alter column c1 set data type char(3);
ERROR:  value too long for type character(3)

postgres=# create table k2 (c1 decimal(20,5), c2 decimal(10,5));
CREATE TABLE
postgres=# insert into k2 values (12345.6, 1.23456),(NULL,NULL),(100012.34,4563.21);
INSERT 0 3
postgres=# alter table k2 alter column c1 set data type decimal(7,2);
ERROR:  numeric field overflow
DETAIL:  A field with precision 7, scale 2 must round to an absolute value less than 10^5.

postgres=# create table k3 (c1 bit(8), c2 varbit(8), c3 text, c4 char(20), c5 varchar(30));
CREATE TABLE
postgres=# insert into k3 values (B'10101010', B'10101010', 'hello', 'world', 'worldhello');
INSERT 0 1
postgres=# alter table k3 alter column c1 set data type bit(4);
ERROR:  bit string length 8 does not match type bit(4)
postgres=# alter table k3 alter column c1 set data type varbit(4);
ERROR:  bit string too long for type bit varying(4)

[spilchen]

tftr!

bors r+

[craig]

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