Consider: ``sql mysql> EXPLAIN SELECT Name FROM Country WHERE Population BETWEEN 1 AND 10000\G ************************ 1. row ************************ id: 1 select_type: SIMPLE table: Country type…

Question

mysql> EXPLAIN SELECT Name FROM Country WHERE Population BETWEEN 1 AND 10000\G
*************************** 1. row ***************************
 id: 1
 select_type: SIMPLE
 table: Country
 type: range
possible_keys: Index_pop
 key: Index_pop
 key_len: 4
 ref: NULL
 rows: 10
 Extra: Using where

Options

• AYou can use an index and return rows that fall within a range of values.
• BThe table will be scanned over a certain range of values.
• CThis type of index uses the range hash.
• DThere is a range of indexes that can be used.

• A
  4% (2)
• B
  8% (4)
• C
  86% (44)
• D
  2% (1)

Explanation

There appears to be an error in the answer key. The marked correct answer (C) is actually wrong - "range hash" is not a real MySQL concept and option C is nonsensical.

The correct answer is A. In MySQL's EXPLAIN output, a type of range means the optimizer used an index to locate rows that fall within a specific range of values - exactly what BETWEEN 1 AND 10000 triggers. The index Index_pop is used to scan only the subset of rows where Population falls in that range, not the full table.

Why the distractors are wrong:

• B is wrong because a full table scan would show type: ALL, not range. The whole point of range is that it avoids scanning the entire table by using an index.
• C is wrong because "range hash" is not a MySQL index type or concept - this is fabricated terminology.
• D is wrong - that would describe the possible_keys column, not the type column. type: range means one specific index access method was chosen and used.

Memory tip: Think of type in EXPLAIN as "how MySQL finds the rows." The hierarchy from worst to best is roughly ALL → index → range → ref → eq_ref → const. range means "I used an index, but I'm scanning a slice of it" - better than a full scan, worse than a point lookup.

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