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Proivide comment for NeonRequest struct

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Konstantin Knizhnik
2024-04-11 17:24:39 +03:00
parent 165a1d7bf1
commit 1b2cfc0259
1 changed files with 8 additions and 2 deletions
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10
pgxn/neon/pagestore_client.h
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@@ -79,14 +79,20 @@ typedef enum {
/*
* supertype of all the Neon*Request structs below
*
* If 'latest' is true, we are requesting the latest page version, and 'lsn'
* In old version of Neon we have 'latest' flag indicating that we are requesting the latest page version, and 'lsn'
* is just a hint to the server that we know there are no versions of the page
* (or relation size, for exists/nblocks requests) later than the 'lsn'.
*
* But it doesn't work for hot-standby replica because it may be not at the latest LSN position.
* So we need to be able to specify upper boundary for LSN which page server can send to us.
* This is why 'latest' flag is replaced with 'horizon'. MAX_LSN=~0 value of 'horizon' means that we are requesting latest version.
* If we need version on exact LSN (for static RO replicas), 'horizon' should be set to 0: in this case range [lsn,lsn] is used by page server.
* Otherwise for hot-standby replica we specify in 'horizon' current replay position.
*/
typedef struct
{
NeonMessageTag tag;
XLogRecPtr horizon; /* uppe boundary for page LSN */
XLogRecPtr horizon; /* upper boundary for page LSN */
XLogRecPtr lsn; /* request page version @ this LSN */
} NeonRequest;