This PostgreSQL extension exposes the internal storage structure of variable-length
datatypes, called varlena.
The function pg_toastinfo describes the storage form of a datum:
nullfor NULLsordinaryfor non-varlena datatypesshort inline varlenafor varlena values up to 126 bytes (1 byte header)long inline varlena, (un)compressedfor varlena values up to 1GiB (4 bytes header)toasted varlena, (un)compressedfor varlena values up to 1GiB stored in TOAST tables- compressed varlenas show the compression method (pglz, lz4) in PG14+
The function pg_toastpointer returns a varlena's chunk_id oid in the
corresponding TOAST table. It returns NULL on non-varlena input.
PostgreSQL versions 9.1 and later are supported.
CREATE EXTENSION toastinfo;
CREATE TABLE t (
a text,
b text
);
INSERT INTO t VALUES ('null', NULL);
INSERT INTO t VALUES ('default', 'default');
ALTER TABLE t ALTER COLUMN b SET STORAGE EXTERNAL;
INSERT INTO t VALUES ('external-10', 'external'); -- short inline varlena
INSERT INTO t VALUES ('external-200', repeat('x', 200)); -- long inline varlena, uncompressed
INSERT INTO t VALUES ('external-10000', repeat('x', 10000)); -- toasted varlena, uncompressed
INSERT INTO t VALUES ('external-1000000', repeat('x', 1000000)); -- toasted varlena, uncompressed
ALTER TABLE t ALTER COLUMN b SET STORAGE EXTENDED;
INSERT INTO t VALUES ('extended-10', 'extended'); -- short inline varlena
INSERT INTO t VALUES ('extended-200', repeat('x', 200)); -- long inline varlena, uncompressed
INSERT INTO t VALUES ('extended-10000', repeat('x', 10000)); -- long inline varlena, compressed (pglz)
INSERT INTO t VALUES ('extended-1000000', repeat('x', 1000000)); -- toasted varlena, compressed (pglz)
ALTER TABLE t ALTER COLUMN b SET COMPRESSION lz4;
INSERT INTO t VALUES ('extended-10000', repeat('x', 10000)); -- long inline varlena, compressed (lz4)
INSERT INTO t VALUES ('extended-1000000', repeat('x', 1000000)); -- toasted varlena, compressed (lz4)
SELECT a, length(b), pg_column_size(b), pg_toastinfo(b), pg_toastpointer(b) FROM t;
a │ length │ pg_column_size │ pg_toastinfo │ pg_toastpointer
──────────────────┼─────────┼────────────────┼────────────────────────────────────────┼─────────────────
null │ ∅ │ ∅ │ null │ ∅
default │ 7 │ 8 │ short inline varlena │ ∅
external-10 │ 8 │ 9 │ short inline varlena │ ∅
external-200 │ 200 │ 204 │ long inline varlena, uncompressed │ ∅
external-10000 │ 10000 │ 10000 │ toasted varlena, uncompressed │ 16427
external-1000000 │ 1000000 │ 1000000 │ toasted varlena, uncompressed │ 16428
extended-10 │ 8 │ 9 │ short inline varlena │ ∅
extended-200 │ 200 │ 204 │ long inline varlena, uncompressed │ ∅
extended-10000 │ 10000 │ 125 │ long inline varlena, compressed (pglz) │ ∅
extended-1000000 │ 1000000 │ 11452 │ toasted varlena, compressed (pglz) │ 16429
extended-10000 │ 10000 │ 58 │ long inline varlena, compressed (lz4) │ ∅
extended-1000000 │ 1000000 │ 3936 │ toasted varlena, compressed (lz4) │ 16430
(12 rows)
Author: Christoph Berg [email protected]
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