// limitExpr在org.h2.command.Parser.parseDelete()中调用过optimize了，所以在这里不用再调用
// 因为limitExpr不会涉及到列，所以也不需要调用mapColumns
@Override
public void prepare() {
if (condition != null) {
condition.mapColumns(tableFilter, 0);
condition = condition.optimize(session);
condition.createIndexConditions(session, tableFilter);
}
// 为什么不能像mapColumns把level设为0，因为getBestPlanItem内部会把level当被除数，所以不行。
PlanItem item = tableFilter.getBestPlanItem(session, 1);
tableFilter.setPlanItem(item);
tableFilter.prepare();
}
public void prepare() {
if (isPrepared) {
// sometimes a subquery is prepared twice (CREATE TABLE AS SELECT)
return;
}
if (SysProperties.CHECK && !checkInit) {
DbException.throwInternalError("not initialized");
}
if (orderList != null) {
sort = prepareOrder(orderList, expressions.size());
orderList = null;
}
for (int i = 0; i < expressions.size(); i++) {
Expression e = expressions.get(i);
expressions.set(i, e.optimize(session));
}
if (condition != null) {
condition = condition.optimize(session);
for (TableFilter f : filters) {
// outer joins: must not add index conditions such as
// "c is null" - example:
// create table parent(p int primary key) as select 1;
// create table child(c int primary key, pc int);
// insert into child values(2, 1);
// select p, c from parent
// left outer join child on p = pc where c is null;
if (!f.isJoinOuter() && !f.isJoinOuterIndirect()) {
condition.createIndexConditions(session, f);
}
}
}
if (isGroupQuery && groupIndex == null && havingIndex < 0 && filters.size() == 1) {
if (condition == null) {
Table t = filters.get(0).getTable();
ExpressionVisitor optimizable = ExpressionVisitor.getOptimizableVisitor(t);
isQuickAggregateQuery = isEverything(optimizable);
}
}
cost = preparePlan();
if (distinct
&& session.getDatabase().getSettings().optimizeDistinct
&& !isGroupQuery
&& filters.size() == 1
&& expressions.size() == 1
&& condition == null) {
Expression expr = expressions.get(0);
expr = expr.getNonAliasExpression();
if (expr instanceof ExpressionColumn) {
Column column = ((ExpressionColumn) expr).getColumn();
int selectivity = column.getSelectivity();
Index columnIndex = topTableFilter.getTable().getIndexForColumn(column);
if (columnIndex != null
&& selectivity != Constants.SELECTIVITY_DEFAULT
&& selectivity < 20) {
// the first column must be ascending
boolean ascending = columnIndex.getIndexColumns()[0].sortType == SortOrder.ASCENDING;
Index current = topTableFilter.getIndex();
// if another index is faster
if (columnIndex.canFindNext()
&& ascending
&& (current == null || current.getIndexType().isScan() || columnIndex == current)) {
IndexType type = columnIndex.getIndexType();
// hash indexes don't work, and unique single column indexes don't work
if (!type.isHash() && (!type.isUnique() || columnIndex.getColumns().length > 1)) {
topTableFilter.setIndex(columnIndex);
isDistinctQuery = true;
}
}
}
}
}
if (sort != null && !isQuickAggregateQuery && !isGroupQuery) {
Index index = getSortIndex();
if (index != null) {
Index current = topTableFilter.getIndex();
if (current.getIndexType().isScan() || current == index) {
topTableFilter.setIndex(index);
if (!topTableFilter.hasInComparisons()) {
// in(select ...) and in(1,2,3) my return the key in another order
sortUsingIndex = true;
}
} else if (index.getIndexColumns().length >= current.getIndexColumns().length) {
IndexColumn[] sortColumns = index.getIndexColumns();
IndexColumn[] currentColumns = current.getIndexColumns();
boolean swapIndex = false;
for (int i = 0; i < currentColumns.length; i++) {
if (sortColumns[i].column != currentColumns[i].column) {
swapIndex = false;
break;
}
if (sortColumns[i].sortType != currentColumns[i].sortType) {
swapIndex = true;
}
}
if (swapIndex) {
topTableFilter.setIndex(index);
sortUsingIndex = true;
}
}
}
}
if (!isQuickAggregateQuery && isGroupQuery && getGroupByExpressionCount() > 0) {
Index index = getGroupSortedIndex();
Index current = topTableFilter.getIndex();
if (index != null && (current.getIndexType().isScan() || current == index)) {
topTableFilter.setIndex(index);
isGroupSortedQuery = true;
}
}
expressionArray = new Expression[expressions.size()];
expressions.toArray(expressionArray);
isPrepared = true;
}