Cockroach: sql (bug): adnum in pg_attrdef and attnum in pg_attribute do not match.

[RichardJCai]

adnum in pg_attrdef should match attnum in pg_attribute.
In CRDB, the numbers can mismatch - this is because pg_attrdef currently uses the colNum (position in Columns slice in TableDescriptor) to generate adnum instead of ordinal_position / ColumnID.

In PG - adnum / attnum should match.

Here's an example where it differs from PG.

Postgres:

richard=# create table t(x INT DEFAULT 1, y INT DEFAULT 1);
richard=# alter table t drop column y; 
richard=# alter table t add column y int default 1;
richard=# select adrelid, adnum from pg_attrdef;
 adrelid | adnum 
---------+-------
   16399 |     1
   16399 |     3
(2 rows)

richard=# select attrelid, attname, attnum from pg_attribute where attrelid = 16399;
 attrelid |           attname            | attnum 
----------+------------------------------+--------
    16399 | tableoid                     |     -6
    16399 | cmax                         |     -5
    16399 | xmax                         |     -4
    16399 | cmin                         |     -3
    16399 | xmin                         |     -2
    16399 | ctid                         |     -1
    16399 | x                            |      1
    16399 | ........pg.dropped.2........ |      2
    16399 | y                            |      3
(9 rows)

CRDB:

[email protected]:58838/movr> create table t(x INT DEFAULT 1, y INT DEFAULT 1);
[email protected]:58838/movr> SET sql_safe_updates = false;
[email protected]:58838/movr> alter table t drop column y;
[email protected]:58838/movr> alter table t add column y int default 1;
[email protected]:58838/movr> select adrelid, adnum from pg_attrdef;
  adrelid | adnum
----------+--------
       59 |     1
       59 |     2 # This column corresponds to y, should be 4.
       59 |     3

[email protected]:58838/movr> select attrelid, attname, attnum from pg_attribute where attrelid = 59;
  attrelid | attname | attnum
-----------+---------+---------
        59 | x       |      1
        59 | rowid   |      3
        59 | y       |      4
(3 rows)

[nvanbenschoten]

How does this issue relate to https://github.com/cockroachdb/cockroach/issues/39787?

Running the example you gave against Postgres does seem to produce the same results:

nathan=# create table t1 (id int PRIMARY KEY, id2 int, id3 int default 0);
CREATE TABLE
nathan=# select table_name, column_name, ordinal_position from information_schema.columns where table_name='t1';
 table_name | column_name | ordinal_position
------------+-------------+------------------
 t1         | id          |                1
 t1         | id2         |                2
 t1         | id3         |                3
(3 rows)

nathan=# alter table t1 drop column id3;
ALTER TABLE
nathan=# select table_name, column_name, ordinal_position from information_schema.columns where table_name='t1';
 table_name | column_name | ordinal_position
------------+-------------+------------------
 t1         | id          |                1
 t1         | id2         |                2
(2 rows)

nathan=# alter table t1 add column id3 int;
ALTER TABLE
nathan=# select table_name, column_name, ordinal_position from information_schema.columns where table_name='t1';
 table_name | column_name | ordinal_position
------------+-------------+------------------
 t1         | id          |                1
 t1         | id2         |                2
 t1         | id3         |                4

[RichardJCai]

@nvanbenschoten Ah if that's the case then this looks correct.
However I think there still might be an inconsistency with regards to adnum in (pg_catalog line 413).
adnum seems to be using the colNum in the table instead of the ColumnID, I believe it should be consistent with attnum.

adnum and attnum both are defined as The number of the column
https://www.postgresql.org/docs/9.5/catalog-pg-attrdef.html
https://www.postgresql.org/docs/9.5/catalog-pg-attribute.html

[nvanbenschoten]

Interesting. For these kinds of issues, it's usually best to present CRDB's output vs. Postgres'. PGs docs aren't always the easiest to understand and often have some ambiguity, but showing a difference in behavior that everyone can reproduce is irrefutable.

Is there a sequence of queries that cause adnum and attnum to differ between CRDB and PG?

[RichardJCai]

Oops forgot to include this example - but heres one where adnum and attnum differ on CRDB but not in Postgres.

Postgres:

richard=# create table t(x INT DEFAULT 1, y INT DEFAULT 1);
richard=# alter table t drop column y; 
richard=# alter table t add column y int default 1;
richard=# select adrelid, adnum from pg_attrdef;
 adrelid | adnum 
---------+-------
   16399 |     1
   16399 |     3
(2 rows)

richard=# select attrelid, attname, attnum from pg_attribute where attrelid = 16399;
 attrelid |           attname            | attnum 
----------+------------------------------+--------
    16399 | tableoid                     |     -6
    16399 | cmax                         |     -5
    16399 | xmax                         |     -4
    16399 | cmin                         |     -3
    16399 | xmin                         |     -2
    16399 | ctid                         |     -1
    16399 | x                            |      1
    16399 | ........pg.dropped.2........ |      2
    16399 | y                            |      3
(9 rows)

CRDB:

[email protected]:58838/movr> create table t(x INT DEFAULT 1, y INT DEFAULT 1);
[email protected]:58838/movr> SET sql_safe_updates = false;
[email protected]:58838/movr> alter table t drop column y;
[email protected]:58838/movr> alter table t add column y int default 1;
[email protected]:58838/movr> select adrelid, adnum from pg_attrdef;
  adrelid | adnum
----------+--------
       59 |     1
       59 |     2 # This column corresponds to y, should be 4.
       59 |     3

[email protected]:58838/movr> select attrelid, attname, attnum from pg_attribute where attrelid = 59;
  attrelid | attname | attnum
-----------+---------+---------
        59 | x       |      1
        59 | rowid   |      3
        59 | y       |      4
(3 rows)

[nvanbenschoten]

Got it, so to confirm, the bug is that pg_attrdef.adnum is implemented using the column number instead of column.ID here, right?
https://github.com/cockroachdb/cockroach/blob/998abbe628d7133932c1beb9240a18c229bab735/pkg/sql/pg_catalog.go#L413

[RichardJCai]

Got it, so to confirm, the bug is that pg_attrdef.adnum is implemented using the column number instead of column.ID here, right?
https://github.com/cockroachdb/cockroach/blob/998abbe628d7133932c1beb9240a18c229bab735/pkg/sql/pg_catalog.go#L413

Yep - planning to fix this in a separate PR for adding LogicalColumnIDs to ColumnDescriptors. LogicalColumnID is added to support swapping the order of columns to allow ALTER COLUMN TYPE (uses a column swap after creating and backfilling a column with the new type)

[RichardJCai]

Fixed by
https://github.com/cockroachdb/cockroach/pull/46992