pdf ocr software

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pdf ocr software

pdf ocr software

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If you use a WHERE clause such as WHERE last_name LIKE '%MA%', the optimizer might just decide to skip the index and do a full scan of the table because it needs to perform a pattern match of the entire LAST_NAME column to retrieve data. The optimizer correctly figures that it will go ahead and look up just the table, instead of having to read both the index and the table values. For example, if a table has 1,000 rows placed in 200 blocks, and you perform a full table scan assuming that the database has set the DB_FILE_MULTIBLOCK_READ_COUNT to 8, you ll incur a total of 25 I/Os to read in the entire table. If your index has a low selectivity, most of the index has to be read first. If your index has 40 leaf blocks and you have to read 90 percent of them to get the indexed data first, your I/O is already at 32. On top of this, you have to incur additional I/O to read the table values. However, a full table scan costs you only 25 I/Os, making that a far more efficient choice than using the index. Be aware that the mere existence of an index on a column doesn t guarantee that it will be used all the time. You ll look at some important principles to make your queries more efficient in the following sections.

You can obtain an idea about the I/O distribution in your system by using the query in Listing 22-13. Listing 22-13. Determining I/O Distribution in the Database SQL> 2 3 4 5 6 7 8 9 10 11 12* SELECT d.name, f.phyrds reads, f.phywrts wrts, (f.readtim / decode(f.phyrds,0,-1,f.phyrds)) readtime, (f.writetim / decode(f.phywrts,0,-1,phywrts)) writetime FROM v$datafile d, v$filestat f WHERE d.file# = f.file# ORDER BY d.name; WRTS ---9 7 READTIME ---------.608695652 .277777778 WRITETIME ---------.222222222 0

If you use SQL functions in the WHERE clause (for example, the SUBSTR, INSTR, TO_DATE, and TO_NUMBER functions), the Oracle optimizer will ignore the index on that column. Make sure you use a functionbased index if you must use a SQL function in the WHERE clause.

NAME READS ----------------------------- ----/pa01/oradata/pa/lol_i_17.dbf 23 /pa01/oradata/pa/lol_i_18.dbf 18 ... SQL>

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Most of your SQL statements will involve multitable joins. Often, improper table-joining strategies doom a query. Here are some pointers regarding joining tables wisely: Using the equi join leads to a more efficient query path than otherwise. Try to use equi joins wherever possible. Performing filtering operations early reduces the number of rows to be joined in later steps. Fop example, a WHERE condition applied early reduces the row source that needs to be joined to another table. The goal is to use the table that has the most selective filter as the driving table, because this means fewer rows are passed to the next step. Join in the order that will produce the least number of rows as output to the parent step.

If there s severe I/O contention in your system, you can undertake some of the following steps, depending on your present database configuration: Increase the number of disks in the storage system. Separate the database and the redo log files. For a large table, use partitions to reduce I/O. Stripe the data either manually or by using a RAID disk-striping system. Invest in cutting-edge technology, such as file caching, to avoid I/O bottlenecks. Consider using the new Automatic Storage Management system, which is discussed in 17.

When you need to calculate multiple aggregates from the same table, avoid writing a separate query for each aggregate. With separate queries, Oracle has to read the entire table for each query. It s

The functions of the parameters that are used with the restrict command (nomodify, nopeer, noquery, notrap) relate to the actions of the NTP control messages, which are identified via the mode value of 6 or 7 in the NTP message header See 3 s NTP Data Messages and NTP Control Messages sections for the meaning of the mode variable and possible actions resulting from the use of control messages, as specified by the operation code in the NTP control message header The meaning of the specific parameters that are used with the restrict command in Listing 5-3 is as follows: Nomodify: Ignore all of the NTP control packets that attempt to modify the state of the timeserver; ie, alter the run-time configuration.

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