php - MySQL relationships vs performance -


i create relationships between tables of databases.

i have read indexes slow write queries , unused indexes useless.

to relationship required column foreign key (i using innodb) can link table's primary key.

some of these indexes never used on search query , used relationship purpose.

i'm using php , yii framework queries , writes code based , checked integrity (unique, exists, etc).

will benefit these relationships if can slow down queries in future in big data context?

this highly depends on queries run on tables.

first: yes, every index slows down write operation, since new data has added @ right place (indexes sorted). if index used, not problem, since highly speeds operations.

but lets have @ example:
lets have table data1(id1,id2,some_text) , 1 table data2(id1,some_more_text). , lets have index (id2,id1) on data1 , index id1 on data2. here have add index id1 on data1 add relationship between data1 , data2. can improve changing index (id2, id1) (id1,id2). since indexes in mysql prefix indexes, can use (id1,id2) index id1 , use foreign key.
loose ability use index ups use field id2, since no prefix of index more.

if add indexes should consider following:

  • what queries run on table?
    if filter multiple columns, should put them in 1 index (mysql uses 1 index per table , query)
  • do need foreign key?
    foreign keys useful make database take care data integrity. should try use existing indexes setup relationship, maybe changing order of multi-column index, if possible.

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