On 7/31/23 04:10, jian he wrote:
hi.
drop table x;
create table x(js jsonb);
insert into x select '{"Hello world":1}'::jsonb;
select pg_column_size(js) from x;-- return 33.
based on src/include/utils/jsonb.h
The key and value part is 20 bytes (is it correct?), Jsonb->vl_len_ is
4 byte, JsonbContainer->header is 4 bytes. That's 28 bytes.
but now on-disk is 33 bytes.
so I am not sure where the remaining bytes are.
I don't claim to understand all this but from jsonb.h
/*
* JsonbValue: In-memory representation of Jsonb. This is a convenient
* deserialized representation, that can easily support using the "val"
* union across underlying types during manipulation. The Jsonb on-disk
* representation has various alignment considerations.
*/
/*
* Key/value pair within an Object.
*
* This struct type is only used briefly while constructing a Jsonb; it is
* *not* the on-disk representation.
*
* Pairs with duplicate keys are de-duplicated. We store the originally
* observed pair ordering for the purpose of removing duplicates in a
* well-defined way (which is "last observed wins").
*/
--
Adrian Klaver
adrian.klaver@xxxxxxxxxxx
