Remove the "snapshot too old" feature.

Remove the old_snapshot_threshold setting and mechanism for producing
the error "snapshot too old", originally added by commit 848ef42b.
Unfortunately it had a number of known problems in terms of correctness
and performance, mostly reported by Andres in the course of his work on
snapshot scalability.  We agreed to remove it, after a long period
without an active plan to fix it.

This is certainly a desirable feature, and someone might propose a new
or improved implementation in the future.

Reported-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CACG%3DezYV%2BEvO135fLRdVn-ZusfVsTY6cH1OZqWtezuEYH6ciQA%40mail.gmail.com
Discussion: https://postgr.es/m/20200401064008.qob7bfnnbu4w5cw4%40alap3.anarazel.de
Discussion: https://postgr.es/m/CA%2BTgmoY%3Daqf0zjTD%2B3dUWYkgMiNDegDLFjo%2B6ze%3DWtpik%2B3XqA%40mail.gmail.com

1 files changed, 0 insertions, 142 deletions

diff --git a/contrib/old_snapshot/time_mapping.c b/contrib/old_snapshot/time_mapping.c
deleted file mode 100644
index 352308cd49b..00000000000
--- a/contrib/old_snapshot/time_mapping.c
+++ /dev/null
@@ -1,142 +0,0 @@
-/*-------------------------------------------------------------------------
- *
- * time_mapping.c
- *	  time to XID mapping information
- *
- * Copyright (c) 2020-2023, PostgreSQL Global Development Group
- *
- *	  contrib/old_snapshot/time_mapping.c
- *-------------------------------------------------------------------------
- */
-#include "postgres.h"
-
-#include "funcapi.h"
-#include "storage/lwlock.h"
-#include "utils/old_snapshot.h"
-#include "utils/snapmgr.h"
-#include "utils/timestamp.h"
-
-/*
- * Backend-private copy of the information from oldSnapshotControl which relates
- * to the time to XID mapping, plus an index so that we can iterate.
- *
- * Note that the length of the xid_by_minute array is given by
- * OLD_SNAPSHOT_TIME_MAP_ENTRIES (which is not a compile-time constant).
- */
-typedef struct
-{
-	int			current_index;
-	int			head_offset;
-	TimestampTz head_timestamp;
-	int			count_used;
-	TransactionId xid_by_minute[FLEXIBLE_ARRAY_MEMBER];
-} OldSnapshotTimeMapping;
-
-#define NUM_TIME_MAPPING_COLUMNS 3
-
-PG_MODULE_MAGIC;
-PG_FUNCTION_INFO_V1(pg_old_snapshot_time_mapping);
-
-static OldSnapshotTimeMapping *GetOldSnapshotTimeMapping(void);
-static HeapTuple MakeOldSnapshotTimeMappingTuple(TupleDesc tupdesc,
-												 OldSnapshotTimeMapping *mapping);
-
-/*
- * SQL-callable set-returning function.
- */
-Datum
-pg_old_snapshot_time_mapping(PG_FUNCTION_ARGS)
-{
-	FuncCallContext *funcctx;
-	OldSnapshotTimeMapping *mapping;
-
-	if (SRF_IS_FIRSTCALL())
-	{
-		MemoryContext oldcontext;
-		TupleDesc	tupdesc;
-
-		funcctx = SRF_FIRSTCALL_INIT();
-		oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
-		mapping = GetOldSnapshotTimeMapping();
-		funcctx->user_fctx = mapping;
-		if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
-			elog(ERROR, "return type must be a row type");
-		funcctx->tuple_desc = tupdesc;
-		MemoryContextSwitchTo(oldcontext);
-	}
-
-	funcctx = SRF_PERCALL_SETUP();
-	mapping = (OldSnapshotTimeMapping *) funcctx->user_fctx;
-
-	while (mapping->current_index < mapping->count_used)
-	{
-		HeapTuple	tuple;
-
-		tuple = MakeOldSnapshotTimeMappingTuple(funcctx->tuple_desc, mapping);
-		++mapping->current_index;
-		SRF_RETURN_NEXT(funcctx, HeapTupleGetDatum(tuple));
-	}
-
-	SRF_RETURN_DONE(funcctx);
-}
-
-/*
- * Get the old snapshot time mapping data from shared memory.
- */
-static OldSnapshotTimeMapping *
-GetOldSnapshotTimeMapping(void)
-{
-	OldSnapshotTimeMapping *mapping;
-
-	mapping = palloc(offsetof(OldSnapshotTimeMapping, xid_by_minute)
-					 + sizeof(TransactionId) * OLD_SNAPSHOT_TIME_MAP_ENTRIES);
-	mapping->current_index = 0;
-
-	LWLockAcquire(OldSnapshotTimeMapLock, LW_SHARED);
-	mapping->head_offset = oldSnapshotControl->head_offset;
-	mapping->head_timestamp = oldSnapshotControl->head_timestamp;
-	mapping->count_used = oldSnapshotControl->count_used;
-	for (int i = 0; i < OLD_SNAPSHOT_TIME_MAP_ENTRIES; ++i)
-		mapping->xid_by_minute[i] = oldSnapshotControl->xid_by_minute[i];
-	LWLockRelease(OldSnapshotTimeMapLock);
-
-	return mapping;
-}
-
-/*
- * Convert one entry from the old snapshot time mapping to a HeapTuple.
- */
-static HeapTuple
-MakeOldSnapshotTimeMappingTuple(TupleDesc tupdesc, OldSnapshotTimeMapping *mapping)
-{
-	Datum		values[NUM_TIME_MAPPING_COLUMNS];
-	bool		nulls[NUM_TIME_MAPPING_COLUMNS];
-	int			array_position;
-	TimestampTz timestamp;
-
-	/*
-	 * Figure out the array position corresponding to the current index.
-	 *
-	 * Index 0 means the oldest entry in the mapping, which is stored at
-	 * mapping->head_offset. Index 1 means the next-oldest entry, which is a
-	 * the following index, and so on. We wrap around when we reach the end of
-	 * the array.
-	 */
-	array_position = (mapping->head_offset + mapping->current_index)
-		% OLD_SNAPSHOT_TIME_MAP_ENTRIES;
-
-	/*
-	 * No explicit timestamp is stored for any entry other than the oldest
-	 * one, but each entry corresponds to 1-minute period, so we can just add.
-	 */
-	timestamp = TimestampTzPlusMilliseconds(mapping->head_timestamp,
-											mapping->current_index * 60000);
-
-	/* Initialize nulls and values arrays. */
-	memset(nulls, 0, sizeof(nulls));
-	values[0] = Int32GetDatum(array_position);
-	values[1] = TimestampTzGetDatum(timestamp);
-	values[2] = TransactionIdGetDatum(mapping->xid_by_minute[array_position]);
-
-	return heap_form_tuple(tupdesc, values, nulls);
-}