- COUNT(*) will include NULLS
- COUNT(column_or_expression) won't.
This means COUNT(any_non_null_column)
will give the same as COUNT(*)
of course because there are no NULL values to cause differences.
Generally, COUNT(*)
should be better because any index can be used because COUNT(column_or_expression)
may not be indexed or SARGable
From ANSI-92 (look for "Scalar expressions 125
")
Case:
a) If COUNT(*) is specified, then the result is the cardinality
of T.
b) Otherwise, let TX be the single-column table that is the
result of applying the <value expression> to each row of T
and eliminating null values. If one or more null values are
eliminated, then a completion condition is raised: warning-
null value eliminated in set function.
The same rules apply to SQL Server and Sybase too at least
Note: COUNT(1) is the same as COUNT(*) because 1 is a non-nullable expression.
Chained and Migrated Rows are explained in the Logical Storage Structures of the Concepts Guide.
A migrated row would be a row who's column data was completely transferred from one block to another due to an update. The original block would essentially only have a "forwarding address" stored for that row.
A chained row would have parts of its column data in multiple blocks. The original block would contain both actual column data and a forwarding address for the rest of it. (You can get rows chained to more than two blocks.)
Both are implemented the same way deep down, so they're really two aspects of the same thing.
Also note that for tables with more than 255 columns, all rows are technically chained - one "row piece" can only contain 255 column values. The chaining can happen in the same block, or with other blocks depending on space availability (and isn't particularly "bad" if all the data ends up in the same block).
The only way, as far as I know, to get accurate data on row chaining is to use:
ANALYZE TABLE your_table [partition (your_part)] LIST CHAINED ROWS
See Listing Chained Rows of Tables and Clusters.
This is potentially expensive, the whole table needs to be scanned. Statistics gathering doesn't fill the CHAIN_CNT
column of the dba_tables
view. (I think it might have at some point, but it doesn't in 11.2 at least.)
You can monitor the table fetch continued row
1 statistic (v$sysstat
) to see if a query is affected by chained or migrated rows, but I don't believe you can have that metric per-session so either you need a quiet system to measure, or the reading will be "noisy".
The Secrets of Oracle Row Chaining and Migration has interesting information about chained and migrated rows, how you measure them, and potential ways of fixing them.
As always, don't go about rebuilding tables or changing storage parameters "just because" you see chained or migrated rows. Do so only if you measure that it's actually causing you performance problems.
1 From Statistics Descriptions:
Number of times a chained or migrated row is encountered during a fetch
Retrieving rows that span more than one block increases the logical I/O by a factor that corresponds to the number of blocks than need to be accessed. Exporting and re-importing may eliminate this problem. Evaluate the settings for the storage parameters PCTFREE and PCTUSED. This problem cannot be fixed if rows are larger than database blocks (for example, if the LONG datatype is used and the rows are extremely large).
See also Table Fetch by Continued Row.
Best Answer
first
andlast
returns the first index and last index.count
returns the number of elements in the collection. These do not necessarily give a correct way of referencing the elements in the collection, as you can have sparse collections, where index values are not contiguous (see below example).I hope you see why both versions fail. These work properly only as long as you have dense collections (where index values are contiguous). With sparse collections, both method fail, and you should use a
WHILE
loop: