Consider the example: Two tables foo and bar, each with a jsonb column.
For foo, there are a million rows where the value of the jsonb is [{"a":123}].
For bar, there are a million rows where the value of the jsonb is [{"very_long_key_not_premature_optimization_at_all":123}]
The json key in bar is 46 characters longer than that in foo. Would the size of bar be 46 million bytes greater than foo?
The bigger the key, the more space to store it. JSONB doesn't do anything special to keys.
# CREATE TABLE foo AS SELECT '{"f":true}'::jsonb FROM generate_series(1,1e6);
SELECT 1000000
# CREATE TABLE bar AS SELECT '{"very_long_key_not_premature_optimization_at_all":true}'::jsonb FROM generate_series(1,1e6);
SELECT 1000000
Now look at the tables
# \dt+ foo;
List of relations
Schema | Name | Type | Owner | Size | Description
--------+------+-------+----------+-------+-------------
public | foo | table | ecarroll | 42 MB |
(1 row)
test=# \dt+ bar;
List of relations
Schema | Name | Type | Owner | Size | Description
--------+------+-------+----------+-------+-------------
public | bar | table | ecarroll | 89 MB |
(1 row)
You can see it in the source here
str = TextDatumGetCString(in_datums[i * 2 + 1]);
len = strlen(str);
v.type = jbvString;
v.val.string.len = len;
You may consider checking out ZSON extension which provides JSONB cross-row dictionary compression
ZSON is a PostgreSQL extension for transparent JSONB compression. Compression is based on a shared dictionary of strings most frequently used in specific JSONB documents (not only keys, but also values, array elements, etc).
In some cases ZSON can save half of your disk space and give you about 10% more TPS. Memory is saved as well. See docs/benchmark.md. Everything depends on your data and workload though. Don't believe any benchmarks, re-check everything on your data, configuration, hardware, workload and PostgreSQL version.
