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You can use this to view the contents of the time to XID mapping which the server maintains when old_snapshot_threshold != -1. Being able to view that information may be interesting for users, and it's definitely useful for figuring out whether the mapping is being maintained correctly. It isn't, so that will need to be fixed in a subsequent commit. Patch by me, reviewed by Thomas Munro, Dilip Kumar, Hamid Akhtar. Discussion: http://postgr.es/m/CA+TgmoY=aqf0zjTD+3dUWYkgMiNDegDLFjo+6ze=Wtpik+3XqA@mail.gmail.com
160 lines
4.6 KiB
C
160 lines
4.6 KiB
C
/*-------------------------------------------------------------------------
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*
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* time_mapping.c
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* time to XID mapping information
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*
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* Copyright (c) 2020, PostgreSQL Global Development Group
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*
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* contrib/old_snapshot/time_mapping.c
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*-------------------------------------------------------------------------
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*/
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#include "postgres.h"
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#include "funcapi.h"
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#include "storage/lwlock.h"
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#include "utils/old_snapshot.h"
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#include "utils/snapmgr.h"
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#include "utils/timestamp.h"
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/*
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* Backend-private copy of the information from oldSnapshotControl which relates
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* to the time to XID mapping, plus an index so that we can iterate.
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*
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* Note that the length of the xid_by_minute array is given by
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* OLD_SNAPSHOT_TIME_MAP_ENTRIES (which is not a compile-time constant).
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*/
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typedef struct
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{
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int current_index;
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int head_offset;
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TimestampTz head_timestamp;
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int count_used;
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TransactionId xid_by_minute[FLEXIBLE_ARRAY_MEMBER];
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} OldSnapshotTimeMapping;
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#define NUM_TIME_MAPPING_COLUMNS 3
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PG_MODULE_MAGIC;
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PG_FUNCTION_INFO_V1(pg_old_snapshot_time_mapping);
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static OldSnapshotTimeMapping *GetOldSnapshotTimeMapping(void);
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static TupleDesc MakeOldSnapshotTimeMappingTupleDesc(void);
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static HeapTuple MakeOldSnapshotTimeMappingTuple(TupleDesc tupdesc,
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OldSnapshotTimeMapping *mapping);
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/*
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* SQL-callable set-returning function.
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*/
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Datum
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pg_old_snapshot_time_mapping(PG_FUNCTION_ARGS)
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{
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FuncCallContext *funcctx;
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OldSnapshotTimeMapping *mapping;
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if (SRF_IS_FIRSTCALL())
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{
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MemoryContext oldcontext;
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funcctx = SRF_FIRSTCALL_INIT();
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oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
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mapping = GetOldSnapshotTimeMapping();
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funcctx->user_fctx = mapping;
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funcctx->tuple_desc = MakeOldSnapshotTimeMappingTupleDesc();
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MemoryContextSwitchTo(oldcontext);
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}
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funcctx = SRF_PERCALL_SETUP();
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mapping = (OldSnapshotTimeMapping *) funcctx->user_fctx;
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while (mapping->current_index < mapping->count_used)
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{
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HeapTuple tuple;
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tuple = MakeOldSnapshotTimeMappingTuple(funcctx->tuple_desc, mapping);
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++mapping->current_index;
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SRF_RETURN_NEXT(funcctx, HeapTupleGetDatum(tuple));
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}
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SRF_RETURN_DONE(funcctx);
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}
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/*
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* Get the old snapshot time mapping data from shared memory.
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*/
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static OldSnapshotTimeMapping *
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GetOldSnapshotTimeMapping(void)
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{
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OldSnapshotTimeMapping *mapping;
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mapping = palloc(offsetof(OldSnapshotTimeMapping, xid_by_minute)
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+ sizeof(TransactionId) * OLD_SNAPSHOT_TIME_MAP_ENTRIES);
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mapping->current_index = 0;
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LWLockAcquire(OldSnapshotTimeMapLock, LW_SHARED);
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mapping->head_offset = oldSnapshotControl->head_offset;
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mapping->head_timestamp = oldSnapshotControl->head_timestamp;
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mapping->count_used = oldSnapshotControl->count_used;
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for (int i = 0; i < OLD_SNAPSHOT_TIME_MAP_ENTRIES; ++i)
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mapping->xid_by_minute[i] = oldSnapshotControl->xid_by_minute[i];
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LWLockRelease(OldSnapshotTimeMapLock);
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return mapping;
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}
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/*
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* Build a tuple descriptor for the pg_old_snapshot_time_mapping() SRF.
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*/
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static TupleDesc
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MakeOldSnapshotTimeMappingTupleDesc(void)
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{
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TupleDesc tupdesc;
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tupdesc = CreateTemplateTupleDesc(NUM_TIME_MAPPING_COLUMNS);
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TupleDescInitEntry(tupdesc, (AttrNumber) 1, "array_offset",
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INT4OID, -1, 0);
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TupleDescInitEntry(tupdesc, (AttrNumber) 2, "end_timestamp",
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TIMESTAMPTZOID, -1, 0);
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TupleDescInitEntry(tupdesc, (AttrNumber) 3, "newest_xmin",
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XIDOID, -1, 0);
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return BlessTupleDesc(tupdesc);
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}
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/*
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* Convert one entry from the old snapshot time mapping to a HeapTuple.
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*/
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static HeapTuple
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MakeOldSnapshotTimeMappingTuple(TupleDesc tupdesc, OldSnapshotTimeMapping *mapping)
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{
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Datum values[NUM_TIME_MAPPING_COLUMNS];
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bool nulls[NUM_TIME_MAPPING_COLUMNS];
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int array_position;
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TimestampTz timestamp;
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/*
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* Figure out the array position corresponding to the current index.
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*
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* Index 0 means the oldest entry in the mapping, which is stored at
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* mapping->head_offset. Index 1 means the next-oldest entry, which is a the
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* following index, and so on. We wrap around when we reach the end of the array.
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*/
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array_position = (mapping->head_offset + mapping->current_index)
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% OLD_SNAPSHOT_TIME_MAP_ENTRIES;
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/*
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* No explicit timestamp is stored for any entry other than the oldest one,
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* but each entry corresponds to 1-minute period, so we can just add.
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*/
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timestamp = TimestampTzPlusMilliseconds(mapping->head_timestamp,
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mapping->current_index * 60000);
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/* Initialize nulls and values arrays. */
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memset(nulls, 0, sizeof(nulls));
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values[0] = Int32GetDatum(array_position);
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values[1] = TimestampTzGetDatum(timestamp);
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values[2] = TransactionIdGetDatum(mapping->xid_by_minute[array_position]);
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return heap_form_tuple(tupdesc, values, nulls);
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}
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