mysql - index usage on union where conditions -


i have following query:

select * t t.a = '1' or t.b = '2' or t.c = '3'

full scan performed there no single index satisy these conditions.

are there way avoid full scans , use indexes?

according documentation, mysql offer "index merge". however, i'm not sure gets used in practice.

first, let me assume have indexes on t(a), t(b), , t(c). then, check if index merge helps.

if not, rephrase query union (not union all) query:

select t.* t t.a = '1'  union select t.* t t.b = '2'  union select t.* t t.c = '3';

each of subqueries should use index. union incur overhead removing duplicates. if know there no duplicates (or not care them), change union union all.


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