Which function is used to map data of any size to a fixed length?

Subscript Operator: []#

The [] operator is used to retrieve the value corresponding to a given key from a map:

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Subscript Operator: []#
Map Functions#
Why do hashing algorithms map data of any size to a fixed length?
Is hashing a fixed length?
What maps a variable length message into a fixed length value or message digest?
What are hash functions used for?

SELECT name_to_age_map['Bob'] AS bob_age;

Map Functions#

cardinality(x) → bigint

Returns the cardinality (size) of the map x.

element_at(map(K, V), key) → V

Returns value for given key, or NULL if the key is not contained in the map.

map() → map<unknown, unknown>#

Returns an empty map.

map(array(K), array(V)) -> map(K, V)#

Returns a map created using the given key/value arrays.

SELECT map(ARRAY[1,3], ARRAY[2,4]); -- {1 -> 2, 3 -> 4}

See also map_agg() and multimap_agg() for creating a map as an aggregation.

map_from_entries(array(row(K, V))) -> map(K, V)#

Returns a map created from the given array of entries.

SELECT map_from_entries(ARRAY[(1, 'x'), (2, 'y')]); -- {1 -> 'x', 2 -> 'y'}

multimap_from_entries(array(row(K, V))) -> map(K, array(V))#

Returns a multimap created from the given array of entries. Each key can be associated with multiple values.

SELECT multimap_from_entries(ARRAY[(1, 'x'), (2, 'y'), (1, 'z')]); -- {1 -> ['x', 'z'], 2 -> ['y']}

map_entries(map(K, V)) -> array(row(K, V))#

Returns an array of all entries in the given map.

SELECT map_entries(MAP(ARRAY[1, 2], ARRAY['x', 'y'])); -- [ROW(1, 'x'), ROW(2, 'y')]

map_concat(map1(K, V), map2(K, V), ..., mapN(K, V)) -> map(K, V)#

Returns the union of all the given maps. If a key is found in multiple given maps, that key’s value in the resulting map comes from the last one of those maps.

map_filter(map(K, V), function(K, V, boolean)) -> map(K, V)#

Constructs a map from those entries of map for which function returns true:

SELECT map_filter(MAP(ARRAY[], ARRAY[]), (k, v) -> true); -- {}
SELECT map_filter(MAP(ARRAY[10, 20, 30], ARRAY['a', NULL, 'c']), (k, v) -> v IS NOT NULL); -- {10 -> a, 30 -> c}
SELECT map_filter(MAP(ARRAY['k1', 'k2', 'k3'], ARRAY[20, 3, 15]), (k, v) -> v > 10); -- {k1 -> 20, k3 -> 15}

map_keys(x(K, V)) -> array(K)#

Returns all the keys in the map x.

map_normalize(x(varchar, double)) -> map(varchar, double)#

Returns the map with the same keys but all non-null values are scaled proportionally so that the sum of values becomes 1. Map entries with null values remain unchanged.

map_values(x(K, V)) -> array(V)#

Returns all the values in the map x.

map_zip_with(map(K, V1), map(K, V2), function(K, V1, V2, V3)) -> map(K, V3)#

Merges the two given maps into a single map by applying function to the pair of values with the same key. For keys only presented in one map, NULL will be passed as the value for the missing key.

SELECT map_zip_with(MAP(ARRAY[1, 2, 3], ARRAY['a', 'b', 'c']), -- {1 -> ad, 2 -> be, 3 -> cf}
                    MAP(ARRAY[1, 2, 3], ARRAY['d', 'e', 'f']),
                    (k, v1, v2) -> concat(v1, v2));
SELECT map_zip_with(MAP(ARRAY['k1', 'k2'], ARRAY[1, 2]), -- {k1 -> ROW(1, null), k2 -> ROW(2, 4), k3 -> ROW(null, 9)}
                    MAP(ARRAY['k2', 'k3'], ARRAY[4, 9]),
                    (k, v1, v2) -> (v1, v2));
SELECT map_zip_with(MAP(ARRAY['a', 'b', 'c'], ARRAY[1, 8, 27]), -- {a -> a1, b -> b4, c -> c9}
                    MAP(ARRAY['a', 'b', 'c'], ARRAY[1, 2, 3]),
                    (k, v1, v2) -> k || CAST(v1/v2 AS VARCHAR));

transform_keys(map(K1, V), function(K1, V, K2)) -> map(K2, V)#

Returns a map that applies function to each entry of map and transforms the keys:

SELECT transform_keys(MAP(ARRAY[], ARRAY[]), (k, v) -> k + 1); -- {}
SELECT transform_keys(MAP(ARRAY [1, 2, 3], ARRAY ['a', 'b', 'c']), (k, v) -> k + 1); -- {2 -> a, 3 -> b, 4 -> c}
SELECT transform_keys(MAP(ARRAY ['a', 'b', 'c'], ARRAY [1, 2, 3]), (k, v) -> v * v); -- {1 -> 1, 4 -> 2, 9 -> 3}
SELECT transform_keys(MAP(ARRAY ['a', 'b'], ARRAY [1, 2]), (k, v) -> k || CAST(v as VARCHAR)); -- {a1 -> 1, b2 -> 2}
SELECT transform_keys(MAP(ARRAY [1, 2], ARRAY [1.0, 1.4]), -- {one -> 1.0, two -> 1.4}
                      (k, v) -> MAP(ARRAY[1, 2], ARRAY['one', 'two'])[k]);

transform_values(map(K, V1), function(K, V1, V2)) -> map(K, V2)#

Returns a map that applies function to each entry of map and transforms the values:

SELECT transform_values(MAP(ARRAY[], ARRAY[]), (k, v) -> v + 1); -- {}
SELECT transform_values(MAP(ARRAY [1, 2, 3], ARRAY [10, 20, 30]), (k, v) -> v + k); -- {1 -> 11, 2 -> 22, 3 -> 33}
SELECT transform_values(MAP(ARRAY [1, 2, 3], ARRAY ['a', 'b', 'c']), (k, v) -> k * k); -- {1 -> 1, 2 -> 4, 3 -> 9}
SELECT transform_values(MAP(ARRAY ['a', 'b'], ARRAY [1, 2]), (k, v) -> k || CAST(v as VARCHAR)); -- {a -> a1, b -> b2}
SELECT transform_values(MAP(ARRAY [1, 2], ARRAY [1.0, 1.4]), -- {1 -> one_1.0, 2 -> two_1.4}
                        (k, v) -> MAP(ARRAY[1, 2], ARRAY['one', 'two'])[k] || '_' || CAST(v AS VARCHAR));

Why do hashing algorithms map data of any size to a fixed length?

The part of a hash function that leads to the digest always being the same size regardless of input length is called the compression function. The compression function is then linked whmacith a domain extender which extends the compression function to allow it to map across any length of input.

Is hashing a fixed length?

Key Takeaways. A hash is a function that meets the encrypted demands needed to solve for a blockchain computation. Hashes are of a fixed length since it makes it nearly impossible to guess the length of the hash if someone was trying to crack the blockchain. The same data will always produce the same hashed value.

What maps a variable length message into a fixed length value or message digest?

Hash functions A variable-length plaintext is “hashed” into a (typically) fixed-length hash value (often called a “message digest” or simply a “hash”). Hash functions are primarily used to provide integrity: if the hash of a plaintext changes, the plaintext itself has changed.

What are hash functions used for?

Hash functions are used for data integrity and often in combination with digital signatures. With a good hash function, even a 1-bit change in a message will produce a different hash (on average, half of the bits change). With digital signatures, a message is hashed and then the hash itself is signed.