sql: creating statistics on tables with indexed NOT NULL virtual columns causes internal error

[mgartner]

CREATE STATISTICS fails with an internal error when invoked on a table that has an index on a NOT NULL virtual column.

For example:

CREATE TABLE t (
  k INT PRIMARY KEY,
  a INT,
  b INT NOT NULL AS (a + 10) VIRTUAL
);

INSERT INTO t VALUES (1, 2);

CREATE STATISTICS s FROM t;

Results in:

ERROR: internal error: Non-nullable column "t:b" with no value! Index scanned was "primary" with the index key columns (k) and the values (1)
SQLSTATE: XX000
DETAIL: stack trace:
/go/src/github.com/cockroachdb/cockroach/pkg/sql/row/fetcher.go:1580: finalizeRow()
/go/src/github.com/cockroachdb/cockroach/pkg/sql/row/fetcher.go:1304: NextRow()
/go/src/github.com/cockroachdb/cockroach/pkg/sql/rowexec/tablereader.go:249: Next()
/go/src/github.com/cockroachdb/cockroach/pkg/sql/rowexec/sampler.go:255: mainLoop()
/go/src/github.com/cockroachdb/cockroach/pkg/sql/rowexec/sampler.go:226: Run()
/go/src/github.com/cockroachdb/cockroach/pkg/sql/flowinfra/flow.go:329: func1()
/usr/local/go/src/runtime/asm_amd64.s:1374: goexit()

I believe the problem is that CREATE STATISTICS is trying to collect samples of the virtual column from the primary index. This is invalid because the primary index does not store virtual computed columns.

[mgartner]

The reproduction mentioned about is fixed by #68312. I'll backport that to 21.1.

However, there's still some cases when stats are requested for virtual columns when they should not be. For example, when the virtual column is included in a multi-column index, creating stats fails with the same error as above:

statement ok
SET CLUSTER SETTING sql.stats.multi_column_collection.enabled = true

statement ok
CREATE TABLE t (
  k INT PRIMARY KEY,
  a INT,
  b INT NOT NULL AS (a + 10) VIRTUAL,
  INDEX (a, b)
);

statement ok
INSERT INTO t VALUES (1, 2);

statement ok
CREATE STATISTICS s FROM t;

Explicitly creating stats on a virtual computed column also fails:

statement ok
CREATE TABLE t (
  k INT PRIMARY KEY,
  a INT,
  b INT NOT NULL AS (a + 10) VIRTUAL
);

statement ok
INSERT INTO t VALUES (1, 2);

statement ok
CREATE STATISTICS s ON b FROM t;

I'll fix these additional cases and backport both fixes to 21.1.