Heaps have one row in sys.partitions, with index_id = 0 for each partition used by the heap. Heap StructuresĪ heap is a table without a clustered index. Therefore, changing from a heap to a clustered index structure or back can take a lot of time and require disk space for reordering data in tempdb. If the table has nonclustered indexes, all the nonclustered indexes must all be recreated whenever the clustered index is changed. REBUILD command to rebuild the heap.Ĭreating or dropping clustered indexes requires rewriting the entire table. Create a clustered index on the heap, and then drop that clustered index.To rebuild a heap to reclaim wasted space: To remove a heap, create a clustered index on the heap. If a table already has a clustered index, drop the clustered index to return the table to a heap. To create a heap, create a table without a clustered index. When scanning a heap, these pointers must be followed which limits read-ahead performance, and can incur additional I/O which reduces scan performance. This introduces fragmentation in the heap. ![]() This creates a forwarded record pointing to the new location of the data, and forwarding pointer has to be written in the page that held that data previously, to indicate the new physical location. If you update a record and the update uses more space in the data pages than they are currently using, the record has to be moved to a data page that has enough free space. In a heap, all rows of the heap must be read to find any row.ĭo not use a heap if the data is frequently updated. A clustered index on the range column will avoid sorting the entire heap.ĭo not use a heap when there are no nonclustered indexes and the table is large, unless you intend to return the entire table content without any specified order. Data must be sorted before it is grouped, and a clustered index on the sorting column could avoid the sorting operation.ĭo not use a heap when ranges of data are frequently queried from the table. A clustered index on the sorting column could avoid the sorting operation.ĭo not use a heap when the data is frequently grouped together. When Not to Use a Heapĭo not use a heap when the data is frequently returned in a sorted order. This can be acceptable when the table is small. ![]() SQL Server cannot seek a RID directly on the heap. If a table is a heap and does not have any nonclustered indexes, then the entire table must be read (a table scan) to find any row. Sometimes data professionals also use heaps when data is always accessed through nonclustered indexes, and the RID is smaller than a clustered index key. Data is retrieved from a heap in order of data pages, but not necessarily the order in which data was inserted.
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