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we get the expected INDEX UNIQUE SCAN, and we can see the function is not being applied. You should always avoid implicit conversions anyway. Always compare apples to apples and oranges to oranges. Another case where this comes up frequently is with dates. We try to query -- find all records for today select * from t where trunc(date_col) = trunc(sysdate); and discover that the index on DATE_COL will not be used. We can either index the TRUNC(DATE_COL) or, perhaps more easily, query using range comparison operators. The following demonstrates the use of greater than and less than on a date. Once we realize that the condition TRUNC(DATE_COL) = TRUNC(SYSDATE) is the same as the condition select * from t where date_col >= trunc(sysdate) and date_col < trunc(sysdate+1) this moves all of the functions to the right-hand side of the equation, allowing us to use the index on DATE_COL (and it has the same exact effect as WHERE TRUNC(DATE_COL) = TRUNC(SYSDATE)). If possible, you should always remove the functions from database columns when they are in the predicate. Not only will doing so allow for more indexes to be considered for use, but it will also reduce the amount of processing the database needs to do. In the preceding case, when we used where date_col >= trunc(sysdate) and date_col < trunc(sysdate+1) the TRUNC values are computed once for the query, and then an index could be used to find just the qualifying values. When we used TRUNC(DATE_COL) = TRUNC(SYSDATE), the TRUNC(DATE_COL) had to be evaluated once per row for every row in the entire table (no indexes).

The index, if used, would actually be slower. I see this often people assume that, of course, an index will always make a query go faster. So, they set up a small table, analyze it, and find that the optimizer doesn t use the index. The optimizer is doing exactly the right thing in this case. Oracle (under the CBO) will use an index only when it makes sense to do so. Consider this example: ops$tkyte@ORA11GR2> create table t 2 ( x, y , primary key (x) ) 3 as 4 select rownum x, object_name 5 from all_objects 6 / Table created. ops$tkyte@ORA11GR2> begin 2 dbms_stats.gather_table_stats 3 ( user, 'T', cascade=>true ); 4 end; 5 / PL/SQL procedure successfully completed.

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If we run a query that needs a relatively small percentage of the table, as follows ops$tkyte@ORA11GR2> set autotrace on explain ops$tkyte@ORA11GR2> select count(y) from t where x < 50; COUNT(Y) ---------49 Execution Plan ---------------------------------------------------------0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=3 Card=1 Bytes=28) 1 0 SORT (AGGREGATE) 2 1 TABLE ACCESS (BY INDEX ROWID) OF 'T' (TABLE) (Cost=3 Card=41 Bytes=1148) 3 2 INDEX (RANGE SCAN) OF 'SYS_C009167' (INDEX (UNIQUE)) (Cost=2 Card=41) it will happily use the index; however, we ll find that when the estimated number of rows to be retrieved via the index crosses a threshold (which varies depending on various optimizer settings, physical statistics, and so on), we ll start to observe a full table scan ops$tkyte@ORA11GR2> select count(y) from t where x < 15000; COUNT(Y) ---------14999 Execution Plan ---------------------------------------------------------0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=57 Card=1 Bytes=28) 1 0 SORT (AGGREGATE) 2 1 TABLE ACCESS (FULL) OF 'T' (TABLE) (Cost=57 Card=14994 Bytes=419832) This example shows the optimizer won t always use an index and, in fact, it makes the right choice in skipping indexes. While tuning your queries, if you discover that an index isn t used when you think it ought to be, don t just force it to be used test and prove first that the index is indeed faster (via elapsed and I/O counts) before overruling the CBO. Reason it out.

void sqlite3_progress_handler( sqlite3*, int frq, int(*)(void*), void*); /* /* /* /* connection handle frequency of callback callback function application data */ */ */ */

The icon set is added to the cell, to the left of the numbers. To remove the numbers from the cells, so only the icons are visible, you can adjust the settings for the conditional formatting you applied. Follow these instructions to remove the numbers from the cells: 1. In the pivot table, select the cells that contain the icon set, and on the Ribbon, click the Home tab, and then in the Styles group, click Conditional Formatting. 2. In the list of conditional formatting options, click Icon Sets, and then click More Rules, at the bottom of the list of Icon Sets. 3. In the New Formatting Rule dialog box, the current settings for the icon set are displayed. At the bottom right, add a check mark to Show Icon Only, and then click OK (see Figure 4-11). You can use the Alignment options on the Ribbon s Home tab to center the icons in the cells.

We haven t analyzed our tables in a while. They used to be small, but now when we look at them, they have grown quite large. An index will now make sense, whereas it didn t originally. If we analyze the table, it will use the index. Without correct statistics, the CBO cannot make the correct decisions.

In my experience, these six cases are the main reasons I find that indexes are not being used. It usually boils down to a case of They cannot be used using them would return incorrect results or They should not be used if they were used, performance would be terrible.

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