Retrieving Data from an Audit Table


The graphs below show the number of reads and CPU (on the Y axis) for all five of these queries for different numbers of rows in the PriceHistories table (on the X axis). Note that both the axes in the graphs are exponential.



It appears that for small sets the string manipulation and the rownumber methods are the most optimal, but as the sets grow larger the cross apply and the subquery(top) perform much better.

Why is this? An explanation of the execution plans

The string manipulation and rownumber queries scan the PriceHistories table, and then join back to Products. Because the PriceHistories table is scanned, the expense of these queries grows more or less linearly with the number of records in the PriceHistories table. Additionally, the string functions in the string manipulation query perform a significant amount of CPU as the amount of data grows larger.

The subquery(top) and cross apply both perform a scan on Products and then a seek on PriceHistories for each product. While at first it is more expensive to do 100 index seeks (200 reads) than do a single scan (6 reads), the expense for index seeks grows logarithmically with the number of records in PriceHistories. As the number of PriceHistories records increases, the performance of these two queries improves relative to the string manipulation and rownumber queries.

The subquery(max) query is consistently one of the worst in terms of reads. It scans the PriceHistories table, like the string manipulation and rownumber queries. However, it then uses an inner loop join to combine the results of the scan with the Products table, forcing 100 index seeks on Products. This is an error on the part of the optimizer, since when a merge join is forced, a single scan is performed on the Products table instead of the index seeks and the number of reads drops by an order of magnitude.


The optimal method to use when retrieving data from a history table depends on the number of historical records per row of current data. In our case, this was the number of rows in the PriceHistories table per row in the Products table. The subquery(max) method consistently performed poorly. The rownumber and string manipulation queries performed best when there were a smaller number of records in the PriceHistories table. The subquery(top) and cross apply queries scaled much better than the other methods.

The subquery(top) and cross apply queries begin to perform better approximately at the point at which performing a seek on the historical table for each row of current data becomes less expensive than performing a single scan on the historical table.  Performing a scan on the historical table requires NumHistRecs / RowsPerPage reads. Performing an index seek costs 1 + log IndexRowsPerPage (NumHistRecs / IndexRowsPerPage ). So the breaking point is approximately when NumHistRecs / RowsPerPage > NumCurRecs * (1 + logIndexRowsPerPage(NumHistRecs/IndexRowsPerPage)). In the above tests, RowsPerPage in PriceHistories is 8000/(4+4+5+8) = 380, NumCurRecs = 100, IndexRowsPerPage = 4+8 = 12, so the performance of subquery(top) or cross apply queries will be better approximately when NumHistRecs / 380 > 100 * (1 + log 12 (NumHistRecs / 12)). This works out to approximately when NumHistRecs > 185,000, or when there is an average of more than 1,850 historical records for each current record. Note, however, that this is an optimized structure. In real world situations, the ratio of the breaking point tends to be significantly lower.


These tests were run on SQL Server 2005 RTM. As always, you should test this on your own systems, since the results may vary due to variations in hardware and version of SQL Server. However, the basic principle remains the same: methods that perform an index seek on the historical table will outperform those that perform a scan as the number of historical records increases.

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