/*
* Select next block to sample.
*
* Uses linear probing algorithm for picking next block.
*/
static BlockNumber
system_rows_nextsampleblock(SampleScanState *node)
{
SystemRowsSamplerData *sampler = (SystemRowsSamplerData *) node->tsm_state;
TableScanDesc scan = node->ss.ss_currentScanDesc;
HeapScanDesc hscan = (HeapScanDesc) scan;
/* First call within scan? */
if (sampler->doneblocks == 0)
{
/* First scan within query? */
if (sampler->step == 0)
{
/* Initialize now that we have scan descriptor */
SamplerRandomState randstate;
/* If relation is empty, there's nothing to scan */
if (hscan->rs_nblocks == 0)
return InvalidBlockNumber;
/* We only need an RNG during this setup step */
sampler_random_init_state(sampler->seed, randstate);
/* Compute nblocks/firstblock/step only once per query */
sampler->nblocks = hscan->rs_nblocks;
/* Choose random starting block within the relation */
/* (Actually this is the predecessor of the first block visited) */
sampler->firstblock = sampler_random_fract(randstate) *
sampler->nblocks;
/* Find relative prime as step size for linear probing */
sampler->step = random_relative_prime(sampler->nblocks, randstate);
}
/* Reinitialize lb */
sampler->lb = sampler->firstblock;
}
/* If we've read all blocks or returned all needed tuples, we're done */
if (++sampler->doneblocks > sampler->nblocks ||
sampler->donetuples >= sampler->ntuples)
return InvalidBlockNumber;
/*
* It's probably impossible for scan->rs_nblocks to decrease between scans
* within a query; but just in case, loop until we select a block number
* less than scan->rs_nblocks. We don't care if scan->rs_nblocks has
* increased since the first scan.
*/
do
{
/* Advance lb, using uint64 arithmetic to forestall overflow */
sampler->lb = ((uint64) sampler->lb + sampler->step) % sampler->nblocks;
} while (sampler->lb >= hscan->rs_nblocks);
return sampler->lb;
}
/*
* Reset state (called by ReScan).
*/
Datum
tsm_system_time_reset(PG_FUNCTION_ARGS)
{
TableSampleDesc *tsdesc = (TableSampleDesc *) PG_GETARG_POINTER(0);
SystemSamplerData *sampler = (SystemSamplerData *) tsdesc->tsmdata;
sampler->lt = InvalidOffsetNumber;
sampler->start_time = GetCurrentTimestamp();
sampler->end_time = TimestampTzPlusMilliseconds(sampler->start_time,
sampler->time);
sampler->estblocks = 2;
sampler->doneblocks = 0;
sampler_random_init_state(sampler->seed, sampler->randstate);
sampler->step = random_relative_prime(sampler->nblocks, sampler->randstate);
sampler->lb = sampler_random_fract(sampler->randstate) * (sampler->nblocks / sampler->step);
PG_RETURN_VOID();
}
/*
* Initializes the state.
*/
Datum
tsm_system_time_init(PG_FUNCTION_ARGS)
{
TableSampleDesc *tsdesc = (TableSampleDesc *) PG_GETARG_POINTER(0);
uint32 seed = PG_GETARG_UINT32(1);
int32 time = PG_ARGISNULL(2) ? -1 : PG_GETARG_INT32(2);
HeapScanDesc scan = tsdesc->heapScan;
SystemSamplerData *sampler;
if (time < 1)
ereport(ERROR,
(errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
errmsg("invalid time limit"),
errhint("Time limit must be positive integer value.")));
sampler = palloc0(sizeof(SystemSamplerData));
/* Remember initial values for reinit */
sampler->seed = seed;
sampler->nblocks = scan->rs_nblocks;
sampler->lt = InvalidOffsetNumber;
sampler->estblocks = 2;
sampler->doneblocks = 0;
sampler->time = time;
sampler->start_time = GetCurrentTimestamp();
sampler->end_time = TimestampTzPlusMilliseconds(sampler->start_time,
sampler->time);
sampler_random_init_state(sampler->seed, sampler->randstate);
/* Find relative prime as step size for linear probing. */
sampler->step = random_relative_prime(sampler->nblocks, sampler->randstate);
/*
* Randomize start position so that blocks close to step size don't have
* higher probability of being chosen on very short scan.
*/
sampler->lb = sampler_random_fract(sampler->randstate) * (sampler->nblocks / sampler->step);
tsdesc->tsmdata = (void *) sampler;
PG_RETURN_VOID();
}
