1. Describe the difference between a procedure, function and anonymous pl/sql block. Candidate should mention use of DECLARE statement, a function must return a value while a procedure doesn?t have to.
2. What is a mutating table error and how can you get around it? This happens with triggers. It occurs because the trigger is trying to update a row it is currently using. The usual fix involves either use of views or temporary tables so the database is selecting from one while updating the other.
3. Describe the use of %ROWTYPE and %TYPE in PL/SQL Expected answer: %ROWTYPE allows you to associate a variable with an entire table row. The %TYPE associates a variable with a single column type.
4. What packages (if any) has Oracle provided for use by developers? Expected answer: Oracle provides the DBMS_ series of packages. There are many which developers should be aware of such as DBMS_SQL, DBMS_PIPE, DBMS_TRANSACTION, DBMS_LOCK, DBMS_ALERT, DBMS_OUTPUT, DBMS_JOB, DBMS_UTILITY, DBMS_DDL, UTL_FILE. If they can mention a few of these and describe how they used them, even better. If they include the SQL routines provided by Oracle, great, but not really what was asked.
5. Describe the use of PL/SQL tables Expected answer: PL/SQL tables are scalar arrays that can be referenced by a binary integer. They can be used to hold values for use in later queries or calculations. In Oracle 8 they will be able to be of the %ROWTYPE designation, or RECORD.
6. When is a declare statement needed ? The DECLARE statement is used in PL/SQL anonymous blocks such as with stand alone, non-stored PL/SQL procedures. It must come first in a PL/SQL stand alone file if it is used.
7. In what order should a open/fetch/loop set of commands in a PL/SQL block be implemented if you use the %NOTFOUND cursor variable in the exit when statement? Why? Expected answer: OPEN then FETCH then LOOP followed by the exit when. If not specified in this order will result in the final return being done twice because of the way the %NOTFOUND is handled by PL/SQL.
8. What are SQLCODE and SQLERRM and why are they important for PL/SQL developers? Expected answer: SQLCODE returns the value of the error number for the last error encountered. The SQLERRM returns the actual error message for the last error encountered. They can be used in exception handling to report, or, store in an error log table, the error that occurred in the code. These are especially useful for the WHEN OTHERS exception.
9. How can you find within a PL/SQL block, if a cursor is open? Expected answer: Use the %ISOPEN cursor status variable.
10. How can you generate debugging output from PL/SQL? Expected answer: Use the DBMS_OUTPUT package. Another possible method is to just use the SHOW ERROR command, but this only shows errors. The DBMS_OUTPUT package can be used to show intermediate results from loops and the status of variables as the procedure is executed. The new package UTL_FILE can also be used.
11. What are the types of triggers? Expected Answer: There are 12 types of triggers in PL/SQL that consist of combinations of the BEFORE, AFTER, ROW, TABLE, INSERT, UPDATE, DELETE and ALL key words: BEFORE ALL ROW INSERT AFTER ALL ROW INSERT BEFORE INSERT AFTER INSERT etc.
Multiple extents in and of themselves aren?t bad. However if you also have chained rows this can hurt performance.
2. How do you set up tablespaces during an Oracle installation?
You should always attempt to use the Oracle Flexible Architecture standard or another partitioning scheme to ensure proper separation of SYSTEM, ROLLBACK, REDO LOG, DATA, TEMPORARY and INDEX segments.
3. You see multiple fragments in the SYSTEM tablespace, what should you check first?
Ensure that users don?t have the SYSTEM tablespace as their TEMPORARY or DEFAULT tablespace assignment by checking the DBA_USERS view.
4. What are some indications that you need to increase the SHARED_POOL_SIZE parameter?
Poor data dictionary or library cache hit ratios, getting error ORA-04031. Another indication is steadily decreasing performance with all other tuning parameters the same.
5. What is the general guideline for sizing db_block_size and db_multi_block_read for an application that does many full table scans?
Oracle almost always reads in 64k chunks. The two should have a product equal to 64 or a multiple of 64.
6. What is the fastest query method for a table
Fetch by rowid
7. Explain the use of TKPROF? What initialization parameter should be turned on to get full TKPROF output?
The tkprof tool is a tuning tool used to determine cpu and execution times for SQL statements. You use it by first setting timed_statistics to true in the initialization file and then turning on tracing for either the entire database via the sql_trace parameter or for the session using the ALTER SESSION command. Once the trace file is generated you run the tkprof tool against the trace file and then look at the output from the tkprof tool. This can also be used to generate explain plan output.
8. When looking at v$sysstat you see that sorts (disk) is high. Is this bad or good? If bad -How do you correct it?
If you get excessive disk sorts this is bad. This indicates you need to tune the sort area parameters in the initialization files. The major sort are parameter is the SORT_AREA_SIZe parameter.
9. When should you increase copy latches? What parameters control copy latches
When you get excessive contention for the copy latches as shown by the "redo copy" latch hit ratio. You can increase copy latches via the initialization parameter LOG_SIMULTANEOUS_COPIES to twice the number of CPUs on your system.
10. Where can you get a list of all initialization parameters for your instance? How about an indication if they are default settings or have been changed
You can look in the init.ora file for an indication of manually set parameters. For all parameters, their value and whether or not the current value is the default value, look in the v$parameter view.
11. Describe hit ratio as it pertains to the database buffers. What is the difference between instantaneous and cumulative hit ratio and which should be used for tuning
The hit ratio is a measure of how many times the database was able to read a value from the buffers verses how many times it had to re-read a data value from the disks. A value greater than 80-90% is good, less could indicate problems. If you simply take the ratio of existing parameters this will be a cumulative value since the database started. If you do a comparison between pairs of readings based on some arbitrary time span, this is the instantaneous ratio for that time span. Generally speaking an instantaneous reading gives more valuable data since it will tell you what your instance is doing for the time it was generated over.
12. Discuss row chaining, how does it happen? How can you reduce it? How do you correct it
Row chaining occurs when a VARCHAR2 value is updated and the length of the new value is longer than the old value and won?t fit in the remaining block space. This results in the row chaining to another block. It can be reduced by setting the storage parameters on the table to appropriate values. It can be corrected by export and import of the effected table.
2. What is a mutating table error and how can you get around it? This happens with triggers. It occurs because the trigger is trying to update a row it is currently using. The usual fix involves either use of views or temporary tables so the database is selecting from one while updating the other.
3. Describe the use of %ROWTYPE and %TYPE in PL/SQL Expected answer: %ROWTYPE allows you to associate a variable with an entire table row. The %TYPE associates a variable with a single column type.
4. What packages (if any) has Oracle provided for use by developers? Expected answer: Oracle provides the DBMS_ series of packages. There are many which developers should be aware of such as DBMS_SQL, DBMS_PIPE, DBMS_TRANSACTION, DBMS_LOCK, DBMS_ALERT, DBMS_OUTPUT, DBMS_JOB, DBMS_UTILITY, DBMS_DDL, UTL_FILE. If they can mention a few of these and describe how they used them, even better. If they include the SQL routines provided by Oracle, great, but not really what was asked.
5. Describe the use of PL/SQL tables Expected answer: PL/SQL tables are scalar arrays that can be referenced by a binary integer. They can be used to hold values for use in later queries or calculations. In Oracle 8 they will be able to be of the %ROWTYPE designation, or RECORD.
6. When is a declare statement needed ? The DECLARE statement is used in PL/SQL anonymous blocks such as with stand alone, non-stored PL/SQL procedures. It must come first in a PL/SQL stand alone file if it is used.
7. In what order should a open/fetch/loop set of commands in a PL/SQL block be implemented if you use the %NOTFOUND cursor variable in the exit when statement? Why? Expected answer: OPEN then FETCH then LOOP followed by the exit when. If not specified in this order will result in the final return being done twice because of the way the %NOTFOUND is handled by PL/SQL.
8. What are SQLCODE and SQLERRM and why are they important for PL/SQL developers? Expected answer: SQLCODE returns the value of the error number for the last error encountered. The SQLERRM returns the actual error message for the last error encountered. They can be used in exception handling to report, or, store in an error log table, the error that occurred in the code. These are especially useful for the WHEN OTHERS exception.
9. How can you find within a PL/SQL block, if a cursor is open? Expected answer: Use the %ISOPEN cursor status variable.
10. How can you generate debugging output from PL/SQL? Expected answer: Use the DBMS_OUTPUT package. Another possible method is to just use the SHOW ERROR command, but this only shows errors. The DBMS_OUTPUT package can be used to show intermediate results from loops and the status of variables as the procedure is executed. The new package UTL_FILE can also be used.
11. What are the types of triggers? Expected Answer: There are 12 types of triggers in PL/SQL that consist of combinations of the BEFORE, AFTER, ROW, TABLE, INSERT, UPDATE, DELETE and ALL key words: BEFORE ALL ROW INSERT AFTER ALL ROW INSERT BEFORE INSERT AFTER INSERT etc.
[edit] Oracle Interview Questions
1. A tablespace has a table with 30 extents in it. Is this bad? Why or why not.Multiple extents in and of themselves aren?t bad. However if you also have chained rows this can hurt performance.
2. How do you set up tablespaces during an Oracle installation?
You should always attempt to use the Oracle Flexible Architecture standard or another partitioning scheme to ensure proper separation of SYSTEM, ROLLBACK, REDO LOG, DATA, TEMPORARY and INDEX segments.
3. You see multiple fragments in the SYSTEM tablespace, what should you check first?
Ensure that users don?t have the SYSTEM tablespace as their TEMPORARY or DEFAULT tablespace assignment by checking the DBA_USERS view.
4. What are some indications that you need to increase the SHARED_POOL_SIZE parameter?
Poor data dictionary or library cache hit ratios, getting error ORA-04031. Another indication is steadily decreasing performance with all other tuning parameters the same.
5. What is the general guideline for sizing db_block_size and db_multi_block_read for an application that does many full table scans?
Oracle almost always reads in 64k chunks. The two should have a product equal to 64 or a multiple of 64.
6. What is the fastest query method for a table
Fetch by rowid
7. Explain the use of TKPROF? What initialization parameter should be turned on to get full TKPROF output?
The tkprof tool is a tuning tool used to determine cpu and execution times for SQL statements. You use it by first setting timed_statistics to true in the initialization file and then turning on tracing for either the entire database via the sql_trace parameter or for the session using the ALTER SESSION command. Once the trace file is generated you run the tkprof tool against the trace file and then look at the output from the tkprof tool. This can also be used to generate explain plan output.
8. When looking at v$sysstat you see that sorts (disk) is high. Is this bad or good? If bad -How do you correct it?
If you get excessive disk sorts this is bad. This indicates you need to tune the sort area parameters in the initialization files. The major sort are parameter is the SORT_AREA_SIZe parameter.
9. When should you increase copy latches? What parameters control copy latches
When you get excessive contention for the copy latches as shown by the "redo copy" latch hit ratio. You can increase copy latches via the initialization parameter LOG_SIMULTANEOUS_COPIES to twice the number of CPUs on your system.
10. Where can you get a list of all initialization parameters for your instance? How about an indication if they are default settings or have been changed
You can look in the init.ora file for an indication of manually set parameters. For all parameters, their value and whether or not the current value is the default value, look in the v$parameter view.
11. Describe hit ratio as it pertains to the database buffers. What is the difference between instantaneous and cumulative hit ratio and which should be used for tuning
The hit ratio is a measure of how many times the database was able to read a value from the buffers verses how many times it had to re-read a data value from the disks. A value greater than 80-90% is good, less could indicate problems. If you simply take the ratio of existing parameters this will be a cumulative value since the database started. If you do a comparison between pairs of readings based on some arbitrary time span, this is the instantaneous ratio for that time span. Generally speaking an instantaneous reading gives more valuable data since it will tell you what your instance is doing for the time it was generated over.
12. Discuss row chaining, how does it happen? How can you reduce it? How do you correct it
Row chaining occurs when a VARCHAR2 value is updated and the length of the new value is longer than the old value and won?t fit in the remaining block space. This results in the row chaining to another block. It can be reduced by setting the storage parameters on the table to appropriate values. It can be corrected by export and import of the effected table.
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