sql: audit all usages of QueryEx to use iterator pattern

This commit audits the usage of `QueryEx` method of the internal
executor in the following manner:
- if the caller only needed to execute the statement, `ExecEx` is now
used
- if the query can return at most one row, then `QueryRowEx` is now used
- if the caller can be refactored to use the iterator pattern, it is
done so.

As a result, almost all usages have been refactored (most notably the
virtual `crdb_internal.jobs` table now uses the iterator pattern, thus
aleviating OOM concerns - the ad-hoc memory accounting logic has been
removed).

`QueryEx` has been renamed to `QueryBufferedEx` to highlight that the
full buffering occurs, and it was added to `sqlutil.InternalExecutor`
interface. The method is now used only in three places.

Release justification: bug fix.

Release note (bug fix): `crdb_internal.jobs` virtual table is now
populated in a paginated fashion, thus, alleviating memory related
concerns when previously we could encounter OOM crash.

pkg/jobs/adopt.go

@@ -265,7 +265,11 @@ func (r *Registry) runJob(
 
 func (r *Registry) servePauseAndCancelRequests(ctx context.Context, s sqlliveness.Session) error {
 	return r.db.Txn(ctx, func(ctx context.Context, txn *kv.Txn) error {
-		it, err := r.ex.QueryIteratorEx(
+		// Note that we have to buffer all rows first - before processing each
+		// job - because we have to make sure that the query executes without an
+		// error (otherwise, the system.jobs table might diverge from the jobs
+		// registry).
+		rows, err := r.ex.QueryBufferedEx(
 			ctx, "cancel/pause-requested", txn, sessiondata.InternalExecutorOverride{User: security.NodeUserName()}, `
 UPDATE system.jobs
 SET status =
@@ -283,20 +287,6 @@ RETURNING id, status`,
 		if err != nil {
 			return errors.Wrap(err, "could not query jobs table")
 		}
-		// Note that we have to buffer all rows first - before processing each
-		// job - because we have to make sure that the query executes without an
-		// error (otherwise, the system.jobs table might diverge from the jobs
-		// registry).
-		// TODO(yuzefovich): use QueryBufferedEx method once it is added to
-		// sqlutil.InternalExecutor interface.
-		var rows []tree.Datums
-		var ok bool
-		for ok, err = it.Next(ctx); ok; ok, err = it.Next(ctx) {
-			rows = append(rows, it.Cur())
-		}
-		if err != nil {
-			return errors.Wrap(err, "could not query jobs table")
-		}
 		for _, row := range rows {
 			id := jobspb.JobID(*row[0].(*tree.DInt))
 			job := &Job{id: id, registry: r}

pkg/kv/kvserver/protectedts/ptstorage/storage_test.go

@@ -704,6 +704,17 @@ func (ie *wrappedInternalExecutor) QueryRowExWithCols(
 	panic("not implemented")
 }
 
+func (ie *wrappedInternalExecutor) QueryBufferedEx(
+	ctx context.Context,
+	opName string,
+	txn *kv.Txn,
+	session sessiondata.InternalExecutorOverride,
+	stmt string,
+	qargs ...interface{},
+) ([]tree.Datums, error) {
+	panic("not implemented")
+}
+
 func (ie *wrappedInternalExecutor) QueryIterator(
 	ctx context.Context, opName string, txn *kv.Txn, stmt string, qargs ...interface{},
 ) (sqlutil.InternalRows, error) {