关系型数据库如何组织数据？

Question

I was thinking that a relational database will store every possible query and the values to return for that query in a hash table.

So like, if each entry in your table had 5 attributes, then you would make a copy of that element for each subset of the 5 attributes that appear in any given query that should return that specific entry. So every individual entry would appear 2^5 = 32 times in the table. This seems like it would be very memory inefficient for large data sets with many entries, but it also allows for the fastest possible query time.

Do real world relational-databases have a mixed version of this where some response time for queries/lookups is traded off for more memory efficiency? If so, how would this be implemented?

Answer 1

That's not how relational databases store data. Keep in mind it's a lot more than 2^32, because you can make queries that have expressions, not simply references to attribute columns. Also queries that are joins, which expands the possibilities immensely.

Even if you could store all possible combinations, it would be a waste because most of them will never be needed.

Instead, databases typically store records, where a record includes all columns of one table. If you run a query that only needs some columns, the DBMS still fetches the whole record, and simply ignores columns that you didn't ask for. Then it evaluates any expressions in your query. And finally returns the result set.

MySQL does not use hash tables to store these records, it uses a B+Tree data structure, so looking up a record by its primary key takes O(log n) time.