It is not uncommon for an application to use WHERE conditions like this:
WHERE status = 'DELETED' OR status IS NULLIf you run EXPLAIN, such conditions typically only cause the type column to be show ref_or_null. Whereas, without the check on NULL, it will be ref.
But does this mean that only a small detail in the execution will change, while the query will still be extremely fast? In case no one told you, any good database professional has one answer that is almost always valid. And that answer is: it depends. I know it’s frustrating, but maybe I can alleviate the frustration by explaining why it depends.
A look at numbers
I created a test table with slightly more than 3.5M rows. I built an index on the columns (a, b), in this order, and another on (b) only. Then I’ve run the following queries, and I felt good.
mysql> SELECT COUNT(*) FROM t WHERE (a = 2 OR a IS NULL) AND (b = 5);
+----------+
| COUNT(*) |
+----------+
| 212 |
+----------+
1 row in set (0.01 sec)
mysql> SELECT COUNT(*) FROM t WHERE (a = 2) AND (b = 5 OR b IS NULL);
+----------+
| COUNT(*) |
+----------+
| 120 |
+----------+
1 row in set (0.01 sec)As you can see, if we add an IS NULL condition to any column in the index, the query remains fast. The problem is when we use IS NULL on multiple columns.
mysql> SELECT COUNT(*) FROM t WHERE (a = 2 OR a IS NULL) AND (b = 5 OR b IS NULL);
+----------+
| COUNT(*) |
+----------+
| 1466664 |
+----------+
1 row in set (1 min 21.32 sec)Blimey! It’s dead slow, is’n it? You may think that this depends on the number of rows. We are selecting a much higher number of rows, perhaps that’s the reason for the slowness. We can test it easily: I ran an UPDATE to replace all NULLs with 0 values, and I repeated the query:
mysql> SELECT COUNT(*) FROM t WHERE (a = 2 OR a IS NULL) AND (b = 5 OR b IS NULL);
+----------+
| COUNT(*) |
+----------+
| 120 |
+----------+
1 row in set (0.02 sec)Very fast, as expected! But, is that slowdown really normal? Let’s see what happens if we look for 0 values:
mysql> SELECT COUNT(*) FROM t WHERE (a = 2 OR a = 0) AND (b = 5 OR b = 0);
+----------+
| COUNT(*) |
+----------+
| 2457536 |
+----------+
1 row in set (1.93 sec)We selected even more rows this time, but the query took less than 2 seconds. Still slow, but it’s a huge improvement: the previous version took 81 seconds!
Query plans
What is the difference between these two queries? The first could not use the right index, because ref_or_null cannot be used on multiple columns. It used the index on (b) instead.:
mysql> EXPLAIN SELECT COUNT(*) FROM t WHERE (a = 2 OR a IS NULL) AND (b = 5 OR b IS NULL) G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: t
partitions: NULL
type: ref_or_null
possible_keys: idx_b,idx_a_b
key: idx_b
key_len: 10
ref: NULL
rows: 1815359
filtered: 11.43
Extra: Using where; Using index
1 row in set, 1 warning (0.00 sec)The other version checks two regular values per column, so it uses range on the right index:
mysql> EXPLAIN SELECT COUNT(*) FROM t WHERE (a = 2 OR a = 0) AND (b = 5 OR b = 0) G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: t
partitions: NULL
type: range
possible_keys: idx_b,idx_a_b
key: idx_a_b
key_len: 10
ref: NULL
rows: 1908763
filtered: 100.00
Extra: Using where; Using index
1 row in set, 1 warning (0.00 sec)A range search can involve multiple columns from the same index. The Extra column confirms that the query is executed by only reading the idx_a_b index.
More details on this optimisation are in the IS NULL optimization page, in MySQL documentation.
Equal followed by range
The solution would be to use a regular value instead of NULL. Something that logically means “nothing”, but it is not NULL. Normally, it is 0, or the empty string '', or the beginning of the UNIX epoch '1970-01-01 00:00:00'.
You may or may not be familiar with this aspect of query optimisation, that I will probably describe in a later post. If you know what I’m talking about, you may at least be glad to know that the “= must precede >” rule works with both the following queries. In other words, it is not affected by the ref_or_null limitations.
mysql> EXPLAIN SELECT COUNT(*) FROM t WHERE (a = 2000 OR a = 0) AND (b > 5 OR b = 0) G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: t
partitions: NULL
type: range
possible_keys: idx_b,idx_a_b
key: idx_a_b
key_len: 10
ref: NULL
rows: 1761559
filtered: 100.00
Extra: Using where; Using index
1 row in set, 1 warning (0.01 sec)
mysql> EXPLAIN SELECT COUNT(*) FROM t WHERE (a = 2000 OR a IS NULL) AND (b > 5 OR b IS NULL) G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: t
partitions: NULL
type: range
possible_keys: idx_b,idx_a_b
key: idx_a_b
key_len: 10
ref: NULL
rows: 4
filtered: 100.00
Extra: Using where; Using index
1 row in set, 1 warning (0.00 sec)See also
Courses:
Conclusions
There are some good reasons to use NULL. For example, Using NULL as default value by FromDual’s Shinguz, argues that using NULL makes tables smaller.
But I believe that most of the times we should avoid anything that makes a good query plan impossible. This is something you may want to think about, before declaring your next column NULLable.
Did you notice any mistake? Do you have thoughts about this article? Please comment!
Federico Razzoli
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