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/*
* Copyright 2004-2019 H2 Group. Multiple-Licensed under the MPL 2.0,
* and the EPL 1.0 (https://h2database.com/html/license.html).
* Initial Developer: H2 Group
*/
package org.h2.command.dml;
import java.util.BitSet;
import java.util.Random;
import java.util.concurrent.TimeUnit;
import org.h2.engine.Session;
import org.h2.expression.Expression;
import org.h2.table.Plan;
import org.h2.table.PlanItem;
import org.h2.table.TableFilter;
import org.h2.util.Permutations;
/**
* The optimizer is responsible to find the best execution plan
* for a given query.
*/
class Optimizer {
private static final int MAX_BRUTE_FORCE_FILTERS = 7;
private static final int MAX_BRUTE_FORCE = 2000;
private static final int MAX_GENETIC = 500;
private long startNs;
private BitSet switched;
// possible plans for filters, if using brute force:
// 1 filter 1 plan
// 2 filters 2 plans
// 3 filters 6 plans
// 4 filters 24 plans
// 5 filters 120 plans
// 6 filters 720 plans
// 7 filters 5040 plans
// 8 filters 40320 plan
// 9 filters 362880 plans
// 10 filters 3628800 filters
private final TableFilter[] filters;
private final Expression condition;
private final Session session;
private Plan bestPlan;
private TableFilter topFilter;
private double cost;
private Random random;
private final AllColumnsForPlan allColumnsSet;
Optimizer(TableFilter[] filters, Expression condition, Session session) {
this.filters = filters;
this.condition = condition;
this.session = session;
allColumnsSet = new AllColumnsForPlan(filters);
}
/**
* How many filter to calculate using brute force. The remaining filters are
* selected using a greedy algorithm which has a runtime of (1 + 2 + ... +
* n) = (n * (n-1) / 2) for n filters. The brute force algorithm has a
* runtime of n * (n-1) * ... * (n-m) when calculating m brute force of n
* total. The combined runtime is (brute force) * (greedy).
*
* @param filterCount the number of filters total
* @return the number of filters to calculate using brute force
*/
private static int getMaxBruteForceFilters(int filterCount) {
int i = 0, j = filterCount, total = filterCount;
while (j > 0 && total * (j * (j - 1) / 2) < MAX_BRUTE_FORCE) {
j--;
total *= j;
i++;
}
return i;
}
private void calculateBestPlan() {
cost = -1;
if (filters.length == 1 || session.isForceJoinOrder()) {
testPlan(filters);
} else {
startNs = System.nanoTime();
if (filters.length <= MAX_BRUTE_FORCE_FILTERS) {
calculateBruteForceAll();
} else {
calculateBruteForceSome();
random = new Random(0);
calculateGenetic();
}
}
}
private void calculateFakePlan() {
cost = -1;
bestPlan = new Plan(filters, filters.length, condition);
}
private boolean canStop(int x) {
return (x & 127) == 0
&& cost >= 0 // don't calculate for simple queries (no rows or so)
&& 10 * (System.nanoTime() - startNs) > cost * TimeUnit.MILLISECONDS.toNanos(1);
}
private void calculateBruteForceAll() {
TableFilter[] list = new TableFilter[filters.length];
Permutations p = Permutations.create(filters, list);
for (int x = 0; !canStop(x) && p.next(); x++) {
testPlan(list);
}
}
private void calculateBruteForceSome() {
int bruteForce = getMaxBruteForceFilters(filters.length);
TableFilter[] list = new TableFilter[filters.length];
Permutations p = Permutations.create(filters, list, bruteForce);
for (int x = 0; !canStop(x) && p.next(); x++) {
// find out what filters are not used yet
for (TableFilter f : filters) {
f.setUsed(false);
}
for (int i = 0; i < bruteForce; i++) {
list[i].setUsed(true);
}
// fill the remaining elements with the unused elements (greedy)
for (int i = bruteForce; i < filters.length; i++) {
double costPart = -1.0;
int bestPart = -1;
for (int j = 0; j < filters.length; j++) {
if (!filters[j].isUsed()) {
if (i == filters.length - 1) {
bestPart = j;
break;
}
list[i] = filters[j];
Plan part = new Plan(list, i+1, condition);
double costNow = part.calculateCost(session, allColumnsSet);
if (costPart < 0 || costNow < costPart) {
costPart = costNow;
bestPart = j;
}
}
}
filters[bestPart].setUsed(true);
list[i] = filters[bestPart];
}
testPlan(list);
}
}
private void calculateGenetic() {
TableFilter[] best = new TableFilter[filters.length];
TableFilter[] list = new TableFilter[filters.length];
for (int x = 0; x < MAX_GENETIC; x++) {
if (canStop(x)) {
break;
}
boolean generateRandom = (x & 127) == 0;
if (!generateRandom) {
System.arraycopy(best, 0, list, 0, filters.length);
if (!shuffleTwo(list)) {
generateRandom = true;
}
}
if (generateRandom) {
switched = new BitSet();
System.arraycopy(filters, 0, best, 0, filters.length);
shuffleAll(best);
System.arraycopy(best, 0, list, 0, filters.length);
}
if (testPlan(list)) {
switched = new BitSet();
System.arraycopy(list, 0, best, 0, filters.length);
}
}
}
private boolean testPlan(TableFilter[] list) {
Plan p = new Plan(list, list.length, condition);
double costNow = p.calculateCost(session, allColumnsSet);
if (cost < 0 || costNow < cost) {
cost = costNow;
bestPlan = p;
return true;
}
return false;
}
private void shuffleAll(TableFilter[] f) {
for (int i = 0; i < f.length - 1; i++) {
int j = i + random.nextInt(f.length - i);
if (j != i) {
TableFilter temp = f[i];
f[i] = f[j];
f[j] = temp;
}
}
}
private boolean shuffleTwo(TableFilter[] f) {
int a = 0, b = 0, i = 0;
for (; i < 20; i++) {
a = random.nextInt(f.length);
b = random.nextInt(f.length);
if (a == b) {
continue;
}
if (a < b) {
int temp = a;
a = b;
b = temp;
}
int s = a * f.length + b;
if (switched.get(s)) {
continue;
}
switched.set(s);
break;
}
if (i == 20) {
return false;
}
TableFilter temp = f[a];
f[a] = f[b];
f[b] = temp;
return true;
}
/**
* Calculate the best query plan to use.
*
* @param parse If we do not need to really get the best plan because it is
* a view parsing stage.
*/
void optimize(boolean parse) {
if (parse) {
calculateFakePlan();
} else {
calculateBestPlan();
bestPlan.removeUnusableIndexConditions();
}
TableFilter[] f2 = bestPlan.getFilters();
topFilter = f2[0];
for (int i = 0; i < f2.length - 1; i++) {
f2[i].addJoin(f2[i + 1], false, null);
}
if (parse) {
return;
}
for (TableFilter f : f2) {
PlanItem item = bestPlan.getItem(f);
f.setPlanItem(item);
}
}
public TableFilter getTopFilter() {
return topFilter;
}
double getCost() {
return cost;
}
}
