 * A table needs to be vacuumed if the number of dead tuples exceeds a
 * threshold.  This threshold is calculated as
 *
 * threshold = vac_base_thresh + vac_scale_factor * reltuples
 * if (threshold > vac_max_thresh)
 *     threshold = vac_max_thresh;
…
	/* Force vacuum if table is at risk of wraparound */
	xidForceLimit = recentXid - freeze_max_age;
	if (xidForceLimit < FirstNormalTransactionId)
		xidForceLimit -= FirstNormalTransactionId;
	relfrozenxid = classForm->relfrozenxid;
	force_vacuum = (TransactionIdIsNormal(relfrozenxid) &&
					TransactionIdPrecedes(relfrozenxid, xidForceLimit));
…
	*wraparound = force_vacuum;
…
		vacthresh = (float4) vac_base_thresh + vac_scale_factor * reltuples;
		if (vac_max_thresh >= 0 && vacthresh > (float4) vac_max_thresh)
			vacthresh = (float4) vac_max_thresh;
…
		/* Determine if this table needs vacuum or analyze. */
		*dovacuum = force_vacuum || (vactuples > vacthresh) ||
			(vac_ins_base_thresh >= 0 && instuples > vacinsthresh);
		*doanalyze = (anltuples > anlthresh);

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