Step by Step solution about How to resolve the algorithm Hash join step by step in the Ruby programming language

Purpose:

The provided Ruby code defines a hashJoin method that performs a hash join between two tables based on specified indexes.

Implementation:

• Hash Phase:

  • The group_by method is used to group the rows of table1 by the values of the column specified by index1.
  • The default value of the hash is set to an empty array to handle cases where a key is not present.

• Join Phase:

  • For each row r in table2, the value at index2 is used as a key to retrieve a list of rows from table1 that have the same value at index1.
  • The rows from table1 and table2 are paired and collected into a single row.
  • The flatten(1) method is used to remove the nested array structure, resulting in a collection of rows.

Example:

The provided example includes two tables:

• table1: Contains two columns: a unique ID and a name.
• table2: Contains two columns: a name and a phobia.

The hashJoin method is called with table1, index1 (1 for the name column), table2, and index2 (0 for the name column). The method returns rows that match the names from table1 and table2.

Output:

The output of the code will be an array of rows containing both the table1 and table2 data:

[[27, "Jonah", "Whales"],
[27, "Jonah", "Spiders"],
[18, "Popeye", "Whales"],
[18, "Popeye", "Spiders"],
[28, "Glory", "Buffy"],
[28, "Alan", "Ghosts"],
[28, "Alan", "Zombies"]]

def hashJoin(table1, index1, table2, index2)
  # hash phase
  h = table1.group_by {|s| s[index1]}
  h.default = []
  # join phase
  table2.collect {|r|
    h[r[index2]].collect {|s| [s, r]}
  }.flatten(1)
end

table1 = [[27, "Jonah"],
          [18, "Alan"],
          [28, "Glory"],
          [18, "Popeye"],
          [28, "Alan"]]
table2 = [["Jonah", "Whales"],
          ["Jonah", "Spiders"],
          ["Alan", "Ghosts"],
          ["Alan", "Zombies"],
          ["Glory", "Buffy"]]

hashJoin(table1, 1, table2, 0).each { |row| p row }