问题
Is there any difference (index selection? speed?) between:
select * from table where x = 'a' or x = 'b'
vs
select * from table where find_in_set(x, 'a,b')
Should I use one of those or are they equal?
回答1:
FIND_IN_SET is a function call which should skip the index altogether.
You should consider using
SELECT * FROM `table` WHERE x IN ('a', 'b')
instead.
回答2:
you should avoid FIND_IN_SET when ever possible! In most cases it is used when columns are not normalized (e.g. csv in a cell). IN is very much faster since it's able to use indexes (if index exists). a crazy example is using FIND_IN_SET(primary_index_column, "1,2,3") using the primary key as first argument. with primary_index_column IN (1,2,3) MySQL is using the primary key as index, which is lightning fast - versus a string function which is very slow. the difference is sometimes 1000x.
e.g. this query
SELECT DISTINCT
a1.usi as usishow,
m1.m1g1
FROM a1 LEFT JOIN p1 ON (a1.usi = p1.id)
left join m1 on (m1.m1id = p1.emxid)
WHERE a1.umi=99999
AND p1.adi = 0 AND
p1.id IN (13700325,13700273,13692054,13691440,13692237,13691503,13691512,13691621,13691632,13691653,13691659,13691682,13691702,13691705,13691720,13691723,13691728,13691763,13691782,13691786,13691815,13691818,13691826,13691828,13691830,13691906,13691831,13691843,13691846,13691848,13691849,13691860,13691873,13691881,13691893,13691945,13691939,13691942,13691977,13692043,13691995,13691996,13692004,13692006,13692011,13692012,13692033,13692090,13692042,13692055,13692062,13692225,13692072,13692089,13692111,13692122,13692142,13692156,13692167,13692170,13692173,13692176,13692178,13692183,13692187,13692193,13692202,13692208,13692217,13692216,13692236,13692238,13692239,13692260,13692274,13692275,13692276,13692280,13692285,13692289,13692291,13692293,13692299,13692305,13692313,13692338,13692348,13692355,13692356,13692365,13692370,13692372,13692376,13692696,13692707,13692701,13692708,13692715,13692727,13692734,13692733,13692735,13692758,13692761,13692762,13692781,13692851,13692854,13694572,13697874,13697878,13697885,13697891,13697897,13697898,13697903,13697904,13697912,13697914,13699899,13697949,13697971,13697976,13698138,13698775,13699851,13699854,13699858,13699859,13699880,13699905,13699908,13699909,13699918,13699932,13699936,13699950,13699959,13699971,13700298,13700162,13700163,13700174,13700177,13700195,13700201,13700238,13700248,13700267,13700276,13700303,13700393,13700399,13700410)
ORDER BY usishow
(containing the three tables a1, m1 and p1. p1.id is the primary index of p1.)
imagine using
FIND_IN_SET(p1.id, 13700325,13700273,...) > 0
using no index, results in 14 sec (!) execution time.
versus
p1.id IN (13700325,13700273,...)
using primary index of p1, results in 0.02 sec (!) execution time.
so, test and
EXPLAIN (EXPLAIN SELECT ....)
all queries using
FIND_IN_SET
with columns that contain single values! It's worth the efford comparing with IN (...) - in most of the cases, you save a lot of time, memory (mayby disk reads) and cpu power!
回答3:
According to my understanding,
select * from table where find_in_set(x, 'a,b')
Would work faster because it searches a value against a list of values and skips the search as soon as it finds a match. But OR works for all the values in the column.
Correct me if i am wrong.
来源：https://stackoverflow.com/questions/18825193/is-there-any-difference-index-selection-speed-in-find-in-set-vs-or