Practical limit to length of SQL query (specifically MySQL) - mysql

Question

Is it particularly bad to have a very, very large SQL query with lots of (potentially redundant) WHERE clauses?
For example, here's a query I've generated from my web application with everything turned off, which should be the largest possible query for this program to generate:
SELECT *
FROM 4e_magic_items
INNER JOIN 4e_magic_item_levels
ON 4e_magic_items.id = 4e_magic_item_levels.itemid
INNER JOIN 4e_monster_sources
ON 4e_magic_items.source = 4e_monster_sources.id
WHERE (itemlevel BETWEEN 1 AND 30)
AND source!=16 AND source!=2 AND source!=5
AND source!=13 AND source!=15 AND source!=3
AND source!=4 AND source!=12 AND source!=7
AND source!=14 AND source!=11 AND source!=10
AND source!=8 AND source!=1 AND source!=6
AND source!=9 AND type!='Arms' AND type!='Feet'
AND type!='Hands' AND type!='Head'
AND type!='Neck' AND type!='Orb'
AND type!='Potion' AND type!='Ring'
AND type!='Rod' AND type!='Staff'
AND type!='Symbol' AND type!='Waist'
AND type!='Wand' AND type!='Wondrous Item'
AND type!='Alchemical Item' AND type!='Elixir'
AND type!='Reagent' AND type!='Whetstone'
AND type!='Other Consumable' AND type!='Companion'
AND type!='Mount' AND (type!='Armor' OR (false ))
AND (type!='Weapon' OR (false ))
ORDER BY type ASC, itemlevel ASC, name ASC
It seems to work well enough, but it's also not particularly high traffic (a few hundred hits a day or so), and I wonder if it would be worth the effort to try and optimize the queries to remove redundancies and such.

Answer 1

Reading your query makes me want to play an RPG.
This is definitely not too long. As long as they are well formatted, I'd say a practical limit is about 100 lines. After that, you're better off breaking subqueries into views just to keep your eyes from crossing.
I've worked with some queries that are 1000+ lines, and that's hard to debug.
By the way, may I suggest a reformatted version? This is mostly to demonstrate the importance of formatting; I trust this will be easier to understand.
select *
from
4e_magic_items mi
,4e_magic_item_levels mil
,4e_monster_sources ms
where mi.id = mil.itemid
and mi.source = ms.id
and itemlevel between 1 and 30
and source not in(16,2,5,13,15,3,4,12,7,14,11,10,8,1,6,9)
and type not in(
'Arms' ,'Feet' ,'Hands' ,'Head' ,'Neck' ,'Orb' ,
'Potion' ,'Ring' ,'Rod' ,'Staff' ,'Symbol' ,'Waist' ,
'Wand' ,'Wondrous Item' ,'Alchemical Item' ,'Elixir' ,
'Reagent' ,'Whetstone' ,'Other Consumable' ,'Companion' ,
'Mount'
)
and ((type != 'Armor') or (false))
and ((type != 'Weapon') or (false))
order by
type asc
,itemlevel asc
,name asc
/*
Some thoughts:
==============
0 - Formatting really matters, in SQL even more than most languages.
1 - consider selecting only the columns you need, not "*"
2 - use of table aliases makes it short & clear ("MI", "MIL" in my example)
3 - joins in the WHERE clause will un-clutter your FROM clause
4 - use NOT IN for long lists
5 - logically, the last two lines can be added to the "type not in" section.
I'm not sure why you have the "or false", but I'll assume some good reason
and leave them here.
*/

Answer 2

Default MySQL 5.0 server limitation is "1MB", configurable up to 1GB.
This is configured via the max_allowed_packet setting on both client and server, and the effective limitation is the lessor of the two.
Caveats:
It's likely that this "packet" limitation does not map directly to characters in a SQL statement. Surely you want to take into account character encoding within the client, some packet metadata, etc.)

Answer 3

SELECT ##global.max_allowed_packet
this is the only real limit it's adjustable on a server so there is no real straight answer

Answer 4

From a practical perspective, I generally consider any SELECT that ends up taking more than 10 lines to write (putting each clause/condition on a separate line) to be too long to easily maintain. At this point, it should probably be done as a stored procedure of some sort, or I should try to find a better way to express the same concept--possibly by creating an intermediate table to capture some relationship I seem to be frequently querying.
Your mileage may vary, and there are some exceptionally long queries that have a good reason to be. But my rule of thumb is 10 lines.
Example (mildly improper SQL):
SELECT x, y, z
FROM a, b
WHERE fiz = 1
AND foo = 2
AND a.x = b.y
AND b.z IN (SELECT q, r, s, t
FROM c, d, e
WHERE c.q = d.r
AND d.s = e.t
AND c.gar IS NOT NULL)
ORDER BY b.gonk
This is probably too large; optimizing, however, would depend largely on context.
Just remember, the longer and more complex the query, the harder it's going to be to maintain.

Answer 5

Most databases support stored procedures to avoid this issue. If your code is fast enough to execute and easy to read, you don't want to have to change it in order to get the compile time down.
An alternative is to use prepared statements so you get the hit only once per client connection and then pass in only the parameters for each call

Answer 6

I'm assuming you mean by 'turned off' that a field doesn't have a value?
Instead of checking if something is not this, and it's also not that etc. can't you just check if the field is null? Or set the field to 'off', and check if type or whatever equals 'off'.