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I have a hstore column in my postgresql database. Each row of the database has a different number of keys/values in the hstore-column. How can i get the number of keys/values for each row?
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In order to consider the key-value pair as a single entity, the PostgreSQL hstore module implements the hstore data type, which can be used in various cases like data that is semi-structured or a row which is having multiple attributes that we cannot try to fetch very often. The data type of keys and values is a string.
The data type of keys and values is a string. The PostgreSQL hstore data type is a similar dictionary we are using with other programming languages; The PostgreSQL hstore is specific to the column. It is not necessary to define the keys beforehand. Explanation: The name of the column whose data type will be store.
Postgresql hstore provides the -> operator to query the value of a specific key from an hstore column. For example, if we want to know ISBN-13 of all available books in the books table, we can use the -> operator as follows:
How does PostgreSQL ROW_NUMBER function work? The ROW_NUMBER () function operates on a set of rows which are termed s a window.mIf the PARTITION BY clause is specified then the row number will increment by one and will start with one.
select hstore_column,
array_length(akeys(hstore_column), 1) as num_keys
from the_table
You can use one of hstore functions, to convert hstore to another data type that can retrieve number of elements. For instance, you can use avalue to count the number of keys on a hstore value:
SELECT id, array_length(avals(my_hstore_field), 1) AS count_keys
FROM mytable;
On the docs there is a nice example to get all the keys and the number of occurrences of it that may be useful for you:
SELECT key, count(*) FROM
(SELECT (each(h)).key FROM testhstore) AS stat
GROUP BY key
ORDER BY count DESC, key;
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