Binary Input and Output¶

Snowflake supports three binary formats or encoding schemes: hex, base64, and UTF-8.

Overview of Supported Binary Formats¶

hex (Default)¶

The "hex" format refers to the hexadecimal, or base 16, system. In this format, each byte is represented by two characters (digits from 0 to 9 and letters from A to F). When using hex to perform conversion:

From:	To:	Notes
Binary	String	hex uses uppercase letters.
String	Binary	hex is case-insensitive.

Hex is the default binary format.

base64¶

The "base64" format encodes binary data (or string data) as printable ASCII characters (letters, digits, and punctuation marks or mathematical operators). (The base64 encoding scheme is defined in RFC 4648 (https://tools.ietf.org/html/rfc4648).)

Base64-encoded data has the following advantages:

Because base64-encoded data is pure ASCII text, it can be stored in systems that support ASCII character data but not BINARY data. For example, binary data that represents music (digital samples), or UTF data that represents Mandarin language characters, can be encoded as ASCII text and stored in systems that support only ASCII characters.
Because base64-encoded data does not contain control characters (e.g. end-of-transmission characters, tab characters), base64-encoded data can be transmitted and received without risk that control characters could be interpreted as commands rather than as data. Base64-encoded data is compatible with older modems and other telecommunications equipment that transmit and receive data one character at a time (without packet headers or protocols that indicate which parts of a packet are data and which are header or control information).

Base64-encoded data has the following disadvantages:

Converting data back and forth between binary and printable ASCII representations consumes computation resources.
Base64-encoded data requires approximately 1/3 more storage space than the original data.

Technical details on the encoding are below for those readers who are interested:

Details of base64 Encoding¶

Each group of three 8-bit bytes (a total of 24 bits) of binary data is re-arranged into four groups of 6 bits each (still 24 bits). Each of the 64 possible combinations of 6 bits is represented by one of the following 64 printable ASCII characters:

uppercase letters (A - Z)
lowercase letters (a - z)
decimal digits (0 - 9)
+
/

In addition, the character = is used for padding if the length of the input is not an exact multiple of 3.

Because base64-encoded data does not contain whitespace characters (e.g. blanks and line breaks), base64-encoded data can be mixed with whitespace if desired. For example, if the transmitter or receiver has a maximum limit on line length, the base64-encoded data can be split into individual lines by adding newline characters without corrupting the data. When using base64 to perform conversion:

From:	To:	Notes
Binary	String	base64 does not insert any whitespace or line breaks.
String	Binary	base64 ignores all whitespace and line breaks.

UTF-8¶

The UTF-8 format refers to the UTF-8 character encoding for Unicode.

UTF-8 is used for text-to-binary encoding. UTF-8 cannot be used for binary-to-text encoding because not all possible BINARY values can be converted to valid UTF-8 strings.

This format is convenient for performing one-to-one conversion between binary and string, for reinterpreting the underlying data as one type or the other rather than actually encoding and decoding.

Session Parameters for Binary Values¶

There are two session parameters that determine how binary values are passed into and out of Snowflake:

BINARY_INPUT_FORMAT:
Specifies the format of VARCHAR input to functions that convert from VARCHAR to BINARY. It is used for:
- Performing conversion to BINARY in the one-argument version of TO_BINARY.
- Loading data into Snowflake (if no file format option is specified; see below for details).
The parameter can be set to 'HEX', 'BASE64', or 'UTF-8' ('UTF8' is also allowed). The parameter values are case-insensitive. The default is 'HEX'.
BINARY_OUTPUT_FORMAT:
Specifies the format of VARCHAR output from functions that convert from BINARY to VARCHAR. It is used for:
- Performing conversion to VARCHAR in the one-argument version of TO_CHAR、TO_VARCHAR.
- Unloading data from Snowflake (if no file format option is specified; see below for details).
- Displaying binary data in human-readable format (e.g. in the Snowflake web interface) when no binary-to-varchar conversion was called explicitly.
The parameter can be set to 'HEX' or 'BASE64'. The parameter values are case-insensitive. The default is 'HEX'.

备注

Because conversion from binary to string can fail with the UTF-8 format, BINARY_OUTPUT_FORMAT can not be set to UTF-8. To use UTF-8 for conversion in this situation, use the two-argument version of TO_CHAR、TO_VARCHAR.

The parameters can be set at the account, user, and session levels. Execute the SHOW PARAMETERS command to view the current parameter settings that apply to all operations in the current session.

File Format Option for Loading/Unloading Binary Values¶

Separate from the binary input and output session parameters, Snowflake provides the BINARY_FORMAT file format option, which can be used to explicitly control binary formatting when loading data into or unloading data from Snowflake tables.

This option can be set to 'HEX', 'BASE64', or 'UTF-8' (values are case-insensitive). The option affects both data loading and unloading and, similar to other file format options, can be specified in a number of ways:

In a named file format, which can then be referenced in a named stage or directly in a COPY command.
In a named stage, which can then be referenced directly in a COPY command.
Directly in a COPY command.

Data Loading¶

When used for data loading, BINARY_FORMAT specifies the format of binary values in your staged data files. This option overrides any value set for the BINARY_INPUT_FORMAT parameter in the session (see Session Parameters for Binary Values above).

If the option is set to 'HEX' or 'BASE64', data loading can fail if the strings in the staged data file are not valid hex or base64. In this case, Snowflake returns an error and then performs the action specified for the ON_ERROR copy option.

Data Unloading¶

When used in data unloading, the BINARY_FORMAT option specifies the format applied to binary values unloaded to the files in the specified stage. This option overrides any value set for the BINARY_OUTPUT_FORMAT parameter in the session (see Session Parameters for Binary Values above).

If the option is set to UTF-8, data unloading fail if any binary values in the table contain invalid UTF-8. In this case, Snowflake returns an error.

Example Input/Output¶

BINARY input/output can be confusing because "what you see is not necessarily what you get".

Consider, for example, the following sequence of code:

CREATE TABLE binary_table (v VARCHAR, b BINARY);
INSERT INTO binary_table (v, b)
    SELECT 'AB', TO_BINARY('AB');

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SELECT v, b FROM binary_table;
+----+----+
| V  | B  |
|----+----|
| AB | AB |
+----+----+

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The outputs for column v (VARCHAR) and column b appear to be identical. Yet the value for column b was converted to binary. Why does the value in column b look unchanged?

The answer is that the argument to TO_BINARY is treated as a sequence of hexadecimal digits (even though it is inside quotes and therefore looks like a string); the 2 characters you see are actually interpreted as a pair of hexadecimal digits that represent 1 byte of binary data, not 2 bytes of string data. (This wouldn't have worked if the input "string" had contained characters other than hexadecimal digits; the result would have been an error message similar to "String '...' is not a legal hex-encoded string".)

Furthermore, when BINARY data is displayed, by default it is displayed as a sequence of hexadecimal digits. Thus the data went in as hexadecimal digits (not a string) and is displayed as hexadecimal digits, so it appears unchanged.

In fact, if the goal was to store the two-character string 'AB', then the code was wrong. The proper code would use the function HEX_ENCODE to convert the string to a sequence of hexadecimal digits (or use another "encode" function to convert to another format, such as base64) before storing the data. Examples of that are below.

Hexadecimal ("HEX") Format Example¶

One way to enter BINARY data is to encode it as a string of hexadecimal characters. An example is below.

Start by creating a table with a BINARY column:
CREATE TABLE demo_binary (b BINARY);
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If you try to insert an "ordinary" string by using the TO_BINARY() function to try to convert it to a valid BINARY value, it fails:
INSERT INTO demo_binary (b) SELECT TO_BINARY('HELP', 'HEX');
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Here's the error message:
100115 (22000): The following string is not a legal hex-encoded value: 'HELP'
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This time, explicitly convert the input to a string of hexadecimal digits before inserting it (this will succeed):
INSERT INTO demo_binary (b) SELECT TO_BINARY(HEX_ENCODE('HELP'), 'HEX');
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Now, retrieve the data:
SELECT TO_VARCHAR(b), HEX_DECODE_STRING(TO_VARCHAR(b)) FROM demo_binary;
+---------------+----------------------------------+
| TO_VARCHAR(B) | HEX_DECODE_STRING(TO_VARCHAR(B)) |
|---------------+----------------------------------|
| 48454C50      | HELP                             |
+---------------+----------------------------------+
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As you can see, by default the output is shown as hexadecimal. To get back the original string, use the function HEX_DECODE_STRING (the complement of the function HEX_ENCODE_STRING that was used earlier to encode the string).

The following helps show in more detail what's going on internally:

SELECT 'HELP', HEX_ENCODE('HELP'), b, HEX_DECODE_STRING(HEX_ENCODE('HELP')),
   TO_VARCHAR(b), HEX_DECODE_STRING(TO_VARCHAR(b)) FROM demo_binary;

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Output:

SELECT 'HELP', HEX_ENCODE('HELP'), b, HEX_DECODE_STRING(HEX_ENCODE('HELP')),
   TO_VARCHAR(b), HEX_DECODE_STRING(TO_VARCHAR(b)) FROM demo_binary;
+--------+--------------------+----------+---------------------------------------+---------------+----------------------------------+
| 'HELP' | HEX_ENCODE('HELP') | B        | HEX_DECODE_STRING(HEX_ENCODE('HELP')) | TO_VARCHAR(B) | HEX_DECODE_STRING(TO_VARCHAR(B)) |
|--------+--------------------+----------+---------------------------------------+---------------+----------------------------------|
| HELP   | 48454C50           | 48454C50 | HELP                                  | 48454C50      | HELP                             |
+--------+--------------------+----------+---------------------------------------+---------------+----------------------------------+

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BASE64 Format Example¶

Before reading this section, you might want to read the Hexadecimal Format section above. The basic concepts are similar, and the Hexadecimal Format section explains them in more detail.

Start by creating a table with a BINARY column:

CREATE TABLE demo_binary (b BINARY);

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Insert a row:

INSERT INTO demo_binary (b) SELECT TO_BINARY(BASE64_ENCODE('HELP'), 'BASE64');

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Retrieve that row:

SELECT 'HELP', BASE64_ENCODE('HELP'),
   BASE64_DECODE_STRING(BASE64_ENCODE('HELP')),
   TO_VARCHAR(b, 'BASE64'), 
   BASE64_DECODE_STRING(TO_VARCHAR(b, 'BASE64'))
   FROM demo_binary;
+--------+-----------------------+---------------------------------------------+-------------------------+-----------------------------------------------+
| 'HELP' | BASE64_ENCODE('HELP') | BASE64_DECODE_STRING(BASE64_ENCODE('HELP')) | TO_VARCHAR(B, 'BASE64') | BASE64_DECODE_STRING(TO_VARCHAR(B, 'BASE64')) |
|--------+-----------------------+---------------------------------------------+-------------------------+-----------------------------------------------|
| HELP   | SEVMUA==              | HELP                                        | SEVMUA==                | HELP                                          |
+--------+-----------------------+---------------------------------------------+-------------------------+-----------------------------------------------+

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UTF-8 Format Example¶

Start by creating a table with a BINARY column:

CREATE TABLE demo_binary (b BINARY);

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Insert a row:

INSERT INTO demo_binary (b) SELECT TO_BINARY('HELP', 'UTF-8');

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Retrieve that row:

SELECT 'HELP', TO_VARCHAR(b, 'UTF-8')
   FROM demo_binary;
+--------+------------------------+
| 'HELP' | TO_VARCHAR(B, 'UTF-8') |
|--------+------------------------|
| HELP   | HELP                   |
+--------+------------------------+

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