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Sun, 29 Mar 2026 05:37:47 +0000 From: Boris Mironov To: "pgsql-docs@lists.postgresql.org" Subject: Request for cleaner explanation of default_toast_compression parameter Thread-Topic: Request for cleaner explanation of default_toast_compression parameter Thread-Index: AQHcvzxyNYWyqqTAjUyZVeEAJn48iQ== Date: Sun, 29 Mar 2026 05:37:47 +0000 Message-ID: Accept-Language: en-CA, en-US Content-Language: en-CA X-MS-Has-Attach: X-MS-TNEF-Correlator: msip_labels: x-ms-exchange-messagesentrepresentingtype: 1 x-ms-publictraffictype: Email x-ms-traffictypediagnostic: PH0PR08MB7020:EE_|DS4PR08MB11086:EE_ x-ms-office365-filtering-correlation-id: df33008b-616b-40cc-94b0-08de8d554f2d x-microsoft-antispam: BCL:0;ARA:14566002|15030799006|15080799012|24071999003|37011999003|8060799015|8062599012|25031999004|31061999003|12050799012|461199028|39105399006|55001999003|20031999003|9400799043|19110799012|10035399007|440099028|3412199025|12091999003|102099032|40105399003; x-microsoft-antispam-message-info: =?iso-8859-1?Q?PJJAnM2/jRB+xFJnfnkbqtA8iTsyufjJEEDPci2rF1Vk/UfwpNyZRo/GiJ?= =?iso-8859-1?Q?UVuDGB5JekUuoEfx48SgFHn1aCswxHi3VncrKSFROLlzOcEIGjcdKSmo2s?= =?iso-8859-1?Q?gH7QG6vnTAw9LPqSvLR36eb3unmwaHwwvE9rG5tLMGPAOkavqV4z/g94BS?= =?iso-8859-1?Q?nQFaWBa6jxx25epczltnuWmmpX6ODb8RzUS+hzef2TkDHMvdU3FZvo9Nln?= =?iso-8859-1?Q?IHzMv7698lGEnDyrvzxwza0TPdGlVNhIJIAqhS0vtAOZ0IvkEVzXqEKhbx?= =?iso-8859-1?Q?YvRgRLaa+bDxwCK3FofzkMmkt+FEoh2y3bSXxeJFRXGv4H7y4Ordvcuqjb?= =?iso-8859-1?Q?ZBsKd+nXjsh9uosu9g8dT19AONAUEC4Eb22Q2YigaHeU82DNiq95YWreDR?= =?iso-8859-1?Q?hXScP5kFaVwQ6+zKKBXQVZdrYvK8/lWPNYGV2IYpelGNTM96VWohq/VsJN?= =?iso-8859-1?Q?0eb14rc7+zTzOwSUzxAYHkzfJm16R9YtlAMBkrbO0yOrMh7Id6PbwNf3Sd?= =?iso-8859-1?Q?g23Rlkc3VVqCzJbUX1gVt28ZOobm5uBH9uLMo5JPtGLTUhvPLdXelcN96p?= =?iso-8859-1?Q?l0F0QEl3G7/e0THU5Iyq33jT1lnUS5qF1610I6Km8Z4ohlWcYdrEj6UaYj?= =?iso-8859-1?Q?6zVoR3JIKGXa9y/5DLQfg8Brj+8GOvv7AMjxWaz9coa4eRMRC9CEHw266n?= =?iso-8859-1?Q?6/+vS37frLpkeqFowmQI1mkBlOcOo+DgI9dMCjjFUIjQ9oY2x4AW8ogF+6?= =?iso-8859-1?Q?k4/2/LJhNP+tbwvRjZ1K8+kMvQNXiHAHnIBt8qPLfBAE80DZl8GrQUik2S?= =?iso-8859-1?Q?GkTmtBQxqZ4XnicZ4LaLiXr9l1mFiRmYa6pIPwsoGxSE6d5PUF8JBEw7uf?= =?iso-8859-1?Q?fgH4KXg/VFEbmWLcS04pL5wTwl8vQ35zKpt8+fGJ413OB+TyUW/T9YGGHW?= =?iso-8859-1?Q?rNxJ3UmkEblnj5fqMlj8dW4bUj4eC1E+CuUuAOSKwwDH16hXzyUf2QWcek?= =?iso-8859-1?Q?3tmjNiSK/8Sma3yojZXPSZ5yr7k0ufroEWQmf1KwmtVC6/Da1Rp9I55zv6?= =?iso-8859-1?Q?AjENnQCCH5SrH76CEZ231Pq4blVDTDSP6Vn/duiprlG2PnEQctTd1Q4qbf?= =?iso-8859-1?Q?BJW8uvUkzyIRZR6CswocwrwVPmW2ua0w5K7Au0am9JmhXpmlEFrwWeLL9c?= =?iso-8859-1?Q?KeCCdr2vFV0rZCj5JY+6a5JgJAun9LeERVEhwfZ6YBXC/HqPt7Vizo8SA/?= =?iso-8859-1?Q?hcsZxK9eX+GE2el9OOpQ=3D=3D?= x-ms-exchange-antispam-messagedata-chunkcount: 1 x-ms-exchange-antispam-messagedata-0: =?iso-8859-1?Q?5SE1eNl+VHPuoAbqpjtWKXJMHC0qM+GUV30w08O9vsN52tGAT6zbffueE1?= =?iso-8859-1?Q?GkTPY3e1J3IAkj/m0J5VU1Hf+B0XbM3ZMO9e2ut6k7ADfPyNWct8Vkmb3F?= =?iso-8859-1?Q?skAMknBA9VapU1/KmEFiTcfCvbtngNS1cO57q2qeywd/F4a0X6cEYUXNoH?= =?iso-8859-1?Q?CyCjT1UQLHjjwyN5VvdESYEMjDRc68HhyeR51mIgyPc4bW8bR7b53hkvXf?= =?iso-8859-1?Q?mk2OHmoI9SnzMDWX/XFSA9ZxvIUHiP1gbuefbacfwMWJ4RVOz+QwQqj5JD?= =?iso-8859-1?Q?xOU5OvxNcwk6RjfiRhzMI+piR4zxuzhj/hOFIqREzuVYw6fPlPhxqnCBr8?= =?iso-8859-1?Q?dQ+0WieMsTUOWsmRGnF9RZvdrHwcOBFiXwc0XiNtxvAZ4SXdIVJYfm+RZH?= =?iso-8859-1?Q?/bJQLAmXFsZG+omgDEDW0ssruQTpDUzB2RiDn4GQKy9NEd101/1zg7NKDj?= =?iso-8859-1?Q?c9ZMUEvZju50g/8DBmac869ppZhElnN4cpusGs/SSOymjCr3m93Y+5jW6o?= =?iso-8859-1?Q?ID/gZSMYGBO6A4Pe+Czh8zm5aZ/YsJ3s991NLgGuH0KtehWfIgoooC5X6A?= =?iso-8859-1?Q?FldS+XzljIqmTujBfyzIUKrr/9Xu5hNsXQkS66KWhH0JEZ1LmHT9G1YC8a?= =?iso-8859-1?Q?zXzzzogC/TGIUHqKfQVgNbOixVt3Vv1gxuBARljghtxBcabx5jpImFRpUf?= =?iso-8859-1?Q?0OR9ITK/igmESYzQ9BcAjiL9GjmvprIMUxwmMDh72pWS6Ed2NauElajA3T?= =?iso-8859-1?Q?Fw0cBDpWpRSXl0QCAqW+QweJ76jvCvZMoC4bNHXh4KKLJWgX/tTgJnlhh5?= =?iso-8859-1?Q?ba+QSqM6THLAdrZk1FeSYlAzMwWb5ghh4iCNG6lhf00nB5Aio6RraXq1Eh?= =?iso-8859-1?Q?267U29IgRR36JTG2u55+ujiEpj/vY/2YNYVrugNfvNzm3sXRj0axaCyNub?= =?iso-8859-1?Q?+B9v/UFDuBN3xHUopg11Yhoxd/HVXbLDvwmv/zzVRjuutxUoDg9EmoHhfy?= =?iso-8859-1?Q?abKBhSeMseHC4mpXx4Fv90nBiAb23zsniC+YokS/1BhUY8rMmK8HGSTXk8?= =?iso-8859-1?Q?lAHBLcfuECzlg92ErjATodzBIbZyKLfNhJTap2+I42K62rTzm/38c4UqaS?= =?iso-8859-1?Q?TrQnocoMdChI59bhLbKjDcIKCKaOOW3VaTK54GnePrtSDztIFI1rN8xhn4?= =?iso-8859-1?Q?CyYxxmz3ljkhhLSOqrXV6CkBhyjm3dPnFW56oB0h4QsElw4OAKpcp8hDMC?= =?iso-8859-1?Q?5wk3TTvwjPkzCBhGfLUaeuvMTX1T3pYA1/rZ3PzSt/3KXTf5LU7Kc43OYz?= =?iso-8859-1?Q?+ZOSOJwRydJZvA2oReJg+ImRQHRfY29BAWSeR7b3i50cKu3USgsILxcpFE?= =?iso-8859-1?Q?re7jX4ubdYW5LFtLQA2ILX72g+4oM+xjpsvUyegBsW2aUtPLLc70u7sGnd?= =?iso-8859-1?Q?ZpQH7QLYngmkH1eHs4OP4y3AYpXvAuRGSYZdIw=3D=3D?= Content-Type: multipart/alternative; boundary="_000_PH0PR08MB7020D7A95CD8F730946B2DA08855APH0PR08MB7020namp_" MIME-Version: 1.0 X-OriginatorOrg: outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-AuthSource: PH0PR08MB7020.namprd08.prod.outlook.com X-MS-Exchange-CrossTenant-RMS-PersistedConsumerOrg: 00000000-0000-0000-0000-000000000000 X-MS-Exchange-CrossTenant-Network-Message-Id: df33008b-616b-40cc-94b0-08de8d554f2d X-MS-Exchange-CrossTenant-originalarrivaltime: 29 Mar 2026 05:37:47.3230 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 84df9e7f-e9f6-40af-b435-aaaaaaaaaaaa X-MS-Exchange-CrossTenant-rms-persistedconsumerorg: 00000000-0000-0000-0000-000000000000 X-MS-Exchange-Transport-CrossTenantHeadersStamped: DS4PR08MB11086 List-Id: List-Help: List-Subscribe: List-Post: List-Owner: List-Archive: Archived-At: Precedence: bulk --_000_PH0PR08MB7020D7A95CD8F730946B2DA08855APH0PR08MB7020namp_ Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Hello, I would like to request a cleaner explanation of default_toast_compression = parameter. It could be just me, but following sentence could be misread: "This variable sets the default TOAST compression method for values of compressible columns." (https://www.postgresql.org/docs/18/runtime-config-client.html#GUC-DEFAULT-= TOAST-COMPRESSION) The way I read it (and perhaps I'm not the only one): "If column value is t= oo big to be stored in-row it goes to TOAST and gets compressed with algori= thm set by default_toast_compression parameter" This leads to the suggestion that initially this column gets compressed by = pglz as default algo and if tuple is bigger than 2000 bytes, the column get= s compressed with algo defined by default_toast_compression and saved into = the TOAST table. This raises suggestion about compressing same string data = twice when default_toast_compression=3Dlz4: 1. in an attempt to save string in-tuple with pglz; 2. with lz4 to save it in the TOAST table. Perhaps something like following will avoid misreading: "This variable sets the default compression method for values of compressib= le columns to be stored in-tuple or TOAST table." Best regards, Boris --_000_PH0PR08MB7020D7A95CD8F730946B2DA08855APH0PR08MB7020namp_ Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
Hello,

I would like to request a cleaner explanation of default_toast_compression = parameter. It could be just me, but following sentence could be misread:

"This variable sets the default TOAST compression method for values of compressible columns."
(https://www.postgresql.org/docs/18/runtime-config-c= lient.html#GUC-DEFAULT-TOAST-COMPRESSION)

The way I read it (and perhaps I'm not the only one): "If column value= is too big to be stored in-row it goes to TOAST and gets compressed with a= lgorithm set by default_toast_compression parameter"

This leads to the suggestion that initially this column gets compressed by = pglz as default algo and if tuple is bigger than 2000 bytes, the column get= s compressed with algo defined by default_toast_compression and saved into = the TOAST table. This raises suggestion about compressing same string data twice when default_toast_compression=3D= lz4:
  1. in an attempt to save string in-tuple with pglz;
  2. with lz4 to save it in the TOAST table.

Perhaps something like following will avoid misreading:

"This variable sets the default compression method for values of compr= essible columns to be stored in-tuple or TOAST table."

Best regards,
Boris

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