Using max function, group by and join - mysql

Question

I have 3 tables:
CREATE TABLE IF NOT EXISTS `disksinfo` (
`idx` int(10) NOT NULL AUTO_INCREMENT,
`hostinfo_idx` int(10) DEFAULT NULL,
`id` char(30) DEFAULT NULL,
`name` char(30) DEFAULT NULL,
`size` bigint(20) DEFAULT NULL,
`freespace` bigint(20) DEFAULT NULL,
PRIMARY KEY (`idx`)
)
CREATE TABLE IF NOT EXISTS `hostinfo` (
`idx` int(10) NOT NULL AUTO_INCREMENT,
`host_idx` int(11) DEFAULT NULL,
`probetime` datetime DEFAULT NULL,
`processor_load` tinyint(4) DEFAULT NULL,
`memory_total` bigint(20) DEFAULT NULL,
`memory_free` bigint(20) DEFAULT NULL,
PRIMARY KEY (`idx`)
)
CREATE TABLE IF NOT EXISTS `hosts` (
`idx` int(10) NOT NULL AUTO_INCREMENT,
`name` char(30) DEFAULT '0',
PRIMARY KEY (`idx`)
)
Basicaly, hosts ist just fixed list of hostnames used in hostinfo table (hostinfo.host_idx = hosts.idx)
hostinfo is a table which is filled each few minutes with data from all hosts and in addition, for each hostinfo row at least one diskinfo row is created. Each diskinfo row contains informations about at least one disk (so, for some hosts there are 3-4 rows of diskinfo). diskinfo.hostinfo_idx = hostinfo.idx.
hostinfo.probetime is simply the time at which data snapshot was created.
What i want to perform now is to select last hostinfo (.probetime) for each particular distinct host (hostinfo.host_idx), while joing informations about disks (diskinfo table) and host names (hosts table)
I came with this:
SELECT hinfo.idx,
hinfo.host_idx,
hinfo.processor_load,
hinfo.memory_total,
hinfo.memory_free,
hnames.idx,
hnames.name,
disks.hostinfo_idx,
disks.id,
disks.name,
disks.size,
disks.freespace,
Max(hinfo.probetime)
FROM systeminfo.hostinfo AS hinfo
INNER JOIN systeminfo.hosts AS hnames
ON hnames.idx = hinfo.host_idx
INNER JOIN systeminfo.disksinfo AS disks
ON disks.hostinfo_idx = hinfo.idx
GROUP BY disks.id,
hnames.name
ORDER BY hnames.name,
disks.id
It seems to work! But, is it 100% correct? Is it optimal? Thanks for any tip!

Answer 1

It's not 100% correct, no.
Suppose you have this table:
x | y | z
-----------------
a b 1
a c 2
d e 1
d f 2
Now when you only group by x, the rows are collapsing and MySQL picks a random row from the collapsed ones. So you might get
x | y | z
-----------------
a b 2
d e 2
or this
x | y | z
-----------------
a c 2
d f 2
Or another combination, this is not determined. Each time you fire your query you might get a different result. The 2 in column z is always there, because of the MAX() function, but you won't necessarily get the corresponding row to it.
Other RDBMSs would actually do the same, but most forbid this by default (in can be forbidden in MySQL, too). You have two possibilities to fix this (actually there are more, but I'll restrict to two).
Either you put all columns you have in your SELECT clause which are not used in an aggregate function like SUM() or MAX() or whatever into the GROUP BY clause as well, like this:
SELECT hinfo.idx,
hinfo.host_idx,
hinfo.processor_load,
hinfo.memory_total,
hinfo.memory_free,
hnames.idx,
hnames.name,
disks.hostinfo_idx,
disks.id,
disks.name,
disks.size,
disks.freespace,
Max(hinfo.probetime)
FROM systeminfo.hostinfo AS hinfo
INNER JOIN systeminfo.hosts AS hnames
ON hnames.idx = hinfo.host_idx
INNER JOIN systeminfo.disksinfo AS disks
ON disks.hostinfo_idx = hinfo.idx
GROUP BY
hinfo.idx,
hinfo.host_idx,
hinfo.processor_load,
hinfo.memory_total,
hinfo.memory_free,
hnames.idx,
hnames.name,
disks.hostinfo_idx,
disks.id,
disks.name,
disks.size,
disks.freespace
ORDER BY hnames.name,
disks.id
Note that this query might get you a different result! I'm just focusing on the problem, that you might get wrong data to the row you think holds the MAX(hinfo.probetime).
Or you solve it like this (and this will get you what you want):
SELECT hinfo.idx,
hinfo.host_idx,
hinfo.processor_load,
hinfo.memory_total,
hinfo.memory_free,
hnames.idx,
hnames.name,
disks.hostinfo_idx,
disks.id,
disks.name,
disks.size,
disks.freespace,
hinfo.probetime
FROM systeminfo.hostinfo AS hinfo
INNER JOIN systeminfo.hosts AS hnames
ON hnames.idx = hinfo.host_idx
INNER JOIN systeminfo.disksinfo AS disks
ON disks.hostinfo_idx = hinfo.idx
WHERE hinfo.probetime = (SELECT MAX(probetime) FROM systeminfo.hostinfo AS hi
INNER JOIN systeminfo.hosts AS hn
ON hnames.idx = hinfo.host_idx
INNER JOIN systeminfo.disksinfo AS d
ON disks.hostinfo_idx = hinfo.idx
WHERE d.id = disks.id AND hn.name = hnames.name)
GROUP BY disks.id,
hnames.name
ORDER BY hnames.name,
disks.id
There's also a nice example in the manual about this: The Rows Holding the Group-wise Maximum of a Certain Column