在主内存中存储关系的最佳方法是什么？

Question

I am working on an application which is a mini DBMS design for evaluating SPJ queries. The program is being implemented in C++.

When I have to process a query for joins and group-by, I need to maintain a set of records in the main memory. Thus, I have to maintain temporary tables in main memory for executing the queries entered by the user.

My question is, what is the best way to achieve this in C++? What data structure do I need to make use of in order to achieve this?

In my application, I am storing data in binary files and using the Catalog (which contains the schema for all the existing tables), I need to retrieve data and process them.

I have only 2 datatypes in my application: int (4 Bytes) and char (1 Byte)

I can use std:: vector. In fact, I tried to use vector of vectors: the inner vector is used for storing attributes, but the problem is there can be many relations existing in the database, and each of them may be any number of attributes. Also, each of these attributes can be either an int or a char. So, I am unable to identify what is the best way to achieve this.

Edit

I cannot use a struct for the tables because I do not know how many columns exist in the newly added tables, since all tables are created at runtime as per the user query. So, a table schema cannot be stored in a struct.

Answer 1

A Relation is a Set of Tuples (and in SQL, a Table is a Bag of Rows). Both in Relational Theory and in SQL, all tuples (/rows) in a relation (/table) "comply to the heading".

So it is interesting to make an object to store relations (/tables) consist of two components: an object of type "Heading" and a Set (/Bag) object containing the actual tuples (/rows).

The "Heading" object is itself a Mapping of attribute (/column) names to "declared data types". I don't know C, but in Java it might be something like Map<AttributeName,TypeName> or Map<AttributeName,Type> or even Map<String,String> (provided you can use those Strings to go get the actual 'Type' objects from wherever they reside).

The set of tuples (/rows) consists of members that are all a Mapping of attribute (/column) names to attribute Values, which are either int or String, in your case. Biggest problem here is that this suggests that you need something like Map<AttributeName,Object>, but you might get into trouble over your int's not being an object.

Answer 2

As a generic container for any table rows, I'd most likely use std::vector (as pointed out by Iarsmans). As for the table columns, I'd most likely define those with structs representing the table schema. For example:

struct DataRow
{
    int col1;
    char col2;
};

typedef std::vector<DataRow> DataTable;
DataTable t;
DataRow dr;
dr.col1 = 1;
dr.col2 = 'a';

t.push_back(dr);