Consider: mysql> EXPLAIN SELECT FROM City WHERE Name = 'Jacksonville' AND CountryCode = 'USA'\G *********************** 1. row ************************ id: 1 select_type: SIMPLE table: city type…

Question

Options

• AIt shows how many bytes will be used from each index row.
• BIt shows the number of characters indexed in the key.
• CIt shows the total size of the index row.
• DIt shows how many columns in the index are examined.

• A
  77% (10)
• B
  8% (1)
• C
  15% (2)

Explanation

key_len reports the number of bytes MySQL will read from each index entry to satisfy the query - it's a byte-length measurement of the index prefix actually used. Here, key_len: 13 means MySQL is consuming 13 bytes of the name_country_index, which corresponds to both the Name and CountryCode columns being fully used (as confirmed by ref: const, const).

Why the distractors are wrong:

• B is wrong because key_len is always in bytes, not characters - a critical distinction when multi-byte charsets (e.g., UTF-8) are involved, where one character can occupy 2–4 bytes.
• C is wrong because key_len only reflects the used portion of the index, not the full index row size (which includes internal overhead MySQL doesn't expose here).
• D is wrong because key_len is a byte count, not a column count - you must infer the number of columns used by knowing each column's byte width and doing the math yourself.

Memory tip: Think of key_len as the "ruler measurement" of how far into the index key MySQL reaches - bigger bytes = more of the index is being used, which generally means a more selective, efficient lookup.

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