TEXT列上的MySQL索引无效

Question

I've got a fairly straightforward table for storing key:value pairs, set up like:

CREATE TABLE `assetProperties` (
  `propertyKey` varchar(255) NOT NULL,
  `propertyValue` text NOT NULL,
  `id` bigint(20) NOT NULL,
  `assetInstance_id` bigint(20) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `FK67F768435C68E1C0` (`id`),
  KEY `FK67F76843FBDFC83F` (`assetInstance_id`),
  KEY `keyIndex` (`propertyKey`),
  KEY `valIndex` (`propertyValue`(255)),
  CONSTRAINT `FK67F76843FBDFC83F` FOREIGN KEY (`assetInstance_id`) REFERENCES `assets` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

...and there are a reasonable number of entries in this table:

mysql> select count(*) from assetProperties;
+----------+
| count(*) |
+----------+
| 19931305 |
+----------+

...and what I want to do is find entries that exactly match a given key:value pair. For example, using a query like:

SELECT count(*) FROM assetProperties WHERE propertyKey = "Wood Species" 
    AND propertyValue = "Jarrah";

Without any indexes on propertyKey and propertyValue , that query runs quite slowly, like on the order of 10+ seconds. Which is to be expected. The problem is, even after adding indexes on both fields, the query time is not improved.

The strange thing is, the index on propertyKey appears to be working correctly:

-- Before adding index
mysql> SELECT count(*) FROM assetProperties WHERE propertyKey = "Wood Species";
+----------+
| count(*) |
+----------+
|   339395 |
+----------+
1 row in set (9.37 sec)  <-- bad

-- After adding index
mysql> SELECT count(*) FROM assetProperties WHERE propertyKey = "Wood Species";
+----------+
| count(*) |
+----------+
|   339395 |
+----------+
1 row in set (0.16 sec)  <-- reasonable

...while the index on propertyValue appears to have made little difference:

-- Before adding index
mysql> SELECT count(*) FROM assetProperties WHERE propertyValue = "Jarrah";
+----------+
| count(*) |
+----------+
|   219099 |
+----------+
1 row in set (12.51 sec)  <-- bad

-- After adding index
mysql> SELECT count(*) FROM assetProperties WHERE propertyValue = "Jarrah";
+----------+
| count(*) |
+----------+
|   219099 |
+----------+
1 row in set (9.75 sec)  <-- still garbage

The only difference between propertyKey and propertyValue is that the former is a varchar(255) column and the latter is text . Is there something preventing MySQL from making efficient use of the index on the text column, or anything I can do to make the query faster?

Edit

Also tried adding a multi-column index across both fields. Made no difference.

And further, if I add a new column (say propertyValueShort ) of type varchar(255) and copy the values from propertyValue into the new column and set up the relevant indexes, it works as it should:

mysql> SELECT count(*) FROM assetProperties WHERE propertyKey = "Wood Species" AND propertyValueShort = "Jarrah";
+----------+
| count(*) |
+----------+
|   219099 |
+----------+
1 row in set (0.14 sec)  <-- acceptable

So that may be the immediate answer (the longest propertyValue currently in use is 88 characters, so I don't really need to be using text there). However still doesn't explain why the index on the text column performs so poorly.

Answer 1

PRIMARY KEY (`id`),
KEY `keyIndex` (`propertyKey`),

-->

PRIMARY KEY(property_key, id),
INDEX(id),

Why?

• InnoDB "clusters" the PK with the data. This means that all rows with the same property_key will now be "next to each other", thereby minimizing disk reads.
• Is id an AUTO_INCREMENT ? If so, INDEX(id) is sufficient; PK is not needed. Else... Why have id ? Can you get rid of it? Not really; you still need to make the PK unique somehow.

What is the ROW_FORMAT of the table? How long is property_value typically? Those impact whether 'big' fields such as TEXT are kept with the row, or stored separately.

(Bottom line: key-value schemas suck.)