Re: Fwd: [PATCH] Add zstd compression for TOAST using extended header format

On Mon, Dec 29, 2025 at 02:45:27PM +0100, Dharin Shah wrote:
> The goal is to centralize “how do we interpret an external datum?” so that
> detoast code paths don’t have to reason directly about va_extinfo encoding
> vs payload layout details. This is intended as groundwork for a follow-up
> patch adding a new vartag-based method (e.g., zstd) without scattering
> special cases in detoast paths.

+static bool
+decode_external_toast_pointer(const struct varlena *attr,
+                           DecodedExternalToast *decoded)
[...]
+typedef enum ToastDecompressMethod
+{
+    TOAST_DECOMP_NONE = 0,
+    TOAST_DECOMP_PGLZ = 1,
+    TOAST_DECOMP_LZ4 = 2
+} ToastDecompressMethod;
+
+typedef struct DecodedExternalToast
+{
+    Oid            toastrelid;
+    Oid            valueid;
+    uint32        rawsize;        /* Decompressed size; for future methods without tcinfo */
+    uint32        extsize;        /* On-disk payload size */
+    ToastDecompressMethod method;
+    uint8        flags;
+} DecodedExternalToast;

Yeah, honestly this is a layer I have been thinking about as well as
one option, but contrary to you I have been focusing on putting that
into varatt.h, with the exception of the value being an Oid8.  I think
that you have an interesting point in focusing your implementation to
be stored in the detoast part, though.  I'd need to spend a bit more
time to see the result this would lead at with the larger 8-byte issue
in mind, but this is something that would come at no real cost as it
has no function pointer redirection compared to what I was first
envisioning on the other thread.  That's especially true if it makes
the CompressionId business easier to mold around when adding a new
vartag.

> Why HAS_TCINFO?
> - Previously, “is compressed?” was used as a proxy for whether the external
> payload begins with tcinfo. This patch makes that explicit: HAS_TCINFO
> captures payload layout, which is distinct from whether the value is
> compressed. This separation is needed for future methods that may store
> external compressed payloads without tcinfo.

It is possible to model the on-memory data as we want.  This
suggestion would be OK with some flags.
--
Michael