SELECT id,foo FROM bar WHERE foo IS NULL;
1001
1002

SELECT id, NVL("FOO", 'null') FROM bar WHERE foo IS NULL;
1001    null
1002    null

CREATE INDEX "BUZ_UTV3"."IX_NULL_FOO"
ON "BUZ_UTV3"."BAR" (NVL("FOO", 'null'))
TABLESPACE "TEST01_BUZUTV3";

SELECT id,foo FROM bar WHERE foo IS NULL;
1001
1002