143 lines
4.2 KiB
C
143 lines
4.2 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) 2020-2023, 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 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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TupleDesc tupdesc;
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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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if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
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elog(ERROR, "return type must be a row type");
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funcctx->tuple_desc = tupdesc;
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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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* 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
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* the following index, and so on. We wrap around when we reach the end of
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* 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
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* one, 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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