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GETRANGE

Introduction​

In Dragonfly, as well as in Redis and Valkey, the GETRANGE command is used to retrieve a substring from a string stored at a specific key. It allows you to extract characters from the stored string by specifying the starting and ending offsets (both inclusive), based on byte positions. This is a useful tool when you want portions of a string without fetching the entire value.

Syntax​

GETRANGE key start end
  • Time complexity: O(N) where N is the length of the returned string. The complexity is ultimately determined by the returned length, but because creating a substring from an existing string is very cheap, it can be considered O(1) for small strings.
  • ACL categories: @read, @string, @slow

Parameter Explanations​

  • key: The key that holds the string from which you want to extract a substring.
  • start: The starting character position (zero-based index) in the string.
  • end: The ending character position (inclusive) for the substring, or -1 to refer to the last character.

Return Values​

The command returns the specified substring extracted from the string value stored at the specified key.

Code Examples​

Basic Example​

Retrieve a substring from a stored string:

dragonfly> SET mykey "example"
OK
dragonfly> GETRANGE mykey 0 2
"exa"

In the above example, the substring exa is returned as it starts from index 0 and ends at index 2.

Extract Substring Using Negative Index​

You can use negative numbers to address characters starting from the end:

dragonfly> SET mykey "example"
OK
dragonfly> GETRANGE mykey -4 -1
"mple"

In this case, -4 refers to the fourth character from the end, and -1 refers to the last character.

Extract a Substring in a Range That Does Not Fully Exist​

If the given range exceeds the string's length, the command will still return what is available:

dragonfly> SET mykey "example"
OK
dragonfly> GETRANGE mykey 5 50
"le"

Here, although the ending index is beyond the actual string length, Dragonfly returns up to the last available characters, which is the le substring.

Use Case for Handling Large Strings​

For user data like URLs, session identifiers, or logs, you may want to extract meaningful segments without loading the entire dataset:

dragonfly> SET logentry "ErrorCode:404,Page:/about,Message:Not Found,ClientIP:192.168.1.50"
OK
dragonfly> GETRANGE logentry 11 24
"404,Page:/about"

This allows you to fetch specific parts of user data efficiently.

Best Practices​

  • Use GETRANGE when dealing with large strings to minimize memory consumption and bandwidth by fetching only what you need.
  • Negative indices are a powerful feature but ensure that they accurately reflect the part of the string you're interested in.

Common Mistakes​

  • Providing out-of-bound indexes does not throw an error, but can lead to unexpected results. Make sure your range matches the string structure.
  • Confusing character positions with byte positions in certain cases, especially with non-ASCII data, may lead to inaccuracies.

FAQs​

What happens if the key does not exist?​

If the key does not exist, GETRANGE will return an empty string.

How does GETRANGE handle out-of-bound indices?​

If the provided start or end indices are out-of-bound (e.g., larger than the string length), the command simply returns as much of the string as falls within the valid range. It does not raise an error for out-of-bound indices.

How does GETRANGE handle encoding?​

GETRANGE operates at the character level for ASCII-compatible encoding, but with multibyte encodings (like UTF-8), ranges refer to byte positions rather than actual characters.

Can I use GETRANGE on binary data?​

Yes, GETRANGE works on strings, whether they are text or binary. The start and end positions relate to bytes when dealing with binary data.