Last updated: Tue Feb 26 23:52:13 EST 2002
+Last updated: Sun Mar 3 11:02:16 EST 2002
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
@@ -72,7 +72,8 @@
get IpcMemoryCreate errors. Why?
3.4) When I try to start postmaster, I
get IpcSemaphoreCreate errors. Why?
- 3.5) How do I control connections from other hosts?
+ 3.5) How do I control connections from other
+ hosts?
3.6) How do I tune the database engine for
better performance?
3.7) What debugging features are available?
@@ -116,9 +117,9 @@
SERIAL insert?
4.15.3) Don't currval() and
nextval() lead to a race condition with other users?
- 4.15.4) Why aren't my sequence numbers reused
- on transaction abort? Why are there gaps in the numbering of my
- sequence/SERIAL column?
+ 4.15.4) Why aren't my sequence numbers
+ reused on transaction abort? Why are there gaps in the numbering of
+ my sequence/SERIAL column?
4.16) What is an OID? What is a
TID?
4.17) What is the meaning of some of the terms
@@ -213,9 +214,9 @@
UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE,
SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
The above is the BSD license, the classic open-source license. It - has no restrictions on how the source code may be used. We like it - and have no intention of changing it.
+The above is the BSD license, the classic open-source license. + It has no restrictions on how the source code may be used. We like + it and have no intention of changing it.
psql has some nice \d commands to show information about @@ -348,9 +351,9 @@
The PostgreSQL book at http://www.PostgreSQL.org/docs/awbook.html - teaches SQL. There is another PostgreSQL book at - - http://www.commandprompt.com/ppbook. + teaches SQL. There is another PostgreSQL book at http://www.commandprompt.com/ppbook. There is a nice tutorial at http://www.intermedia.net/support/sql/sqltut.shtm, at 4.6) How much database disk space is required to store data from a typical text file? -
A PostgreSQL database may require up to five times the disk space - to store data from a text file.
+A PostgreSQL database may require up to five times the disk + space to store data from a text file.
As an example, consider a file of 100,000 lines with an integer - and text description on each line. Suppose the text string avergages - twenty bytes in length. The flat file would be 2.8 MB. The size - of the PostgreSQL database file containing this data can be - estimated as 6.4 MB:
+ and text description on each line. Suppose the text string + avergages twenty bytes in length. The flat file would be 2.8 MB. + The size of the PostgreSQL database file containing this data can + be estimated as 6.4 MB:
36 bytes: each row header (approximate)
24 bytes: one int field and one text filed
@@ -899,33 +902,33 @@
4.8) My queries are slow or don't make use of
the indexes. Why?
+ Indexes are not automatically used by every query. Indexes are only
+ used if the table is larger than a minimum size, and the index
+ selects only a small percentage of the rows in the table. This is
+ because the random disk access caused by an index scan is sometimes
+ slower than a straight read through the table, or sequential scan.
- PostgreSQL does not automatically maintain statistics.
- VACUUM must be run to update the statistics. After
- statistics are updated, the optimizer knows how many rows in the
- table, and can better decide if it should use indexes. Note that
- the optimizer does not use indexes in cases when the table is small
- because a sequential scan would be faster.
+ To determine if an index should be used, PostgreSQL must have
+ statistics about the table. These statistics are collected using
+ VACUUM ANALYZE, or simply ANALYZE.
+ Using statistics, the optimizer knows how many rows are in the
+ table, and can better determine if indexes should be used.
+ Statistics are also valuable in determining optimal join order and
+ join methods. Statistics collection should be performed
+ periodically as the contents of the table change.
- For column-specific optimization statistics, use VACUUM
- ANALYZE. VACUUM ANALYZE is important for
- complex multijoin queries, so the optimizer can estimate the number
- of rows returned from each table, and choose the proper join order.
- The backend does not keep track of column statistics on its own, so
- VACUUM ANALYZE must be run to collect them
- periodically.
-
- Indexes are usually not used for ORDER BY or
- joins. A sequential scan followed by an explicit sort is faster
- than an indexscan of all tuples of a large table. This is because
- random disk access is very slow.
+ Indexes are normally not used for ORDER BY or to
+ perform joins. A sequential scan followed by an explicit sort is
+ usually faster than an index scan of a large table.
+ However, LIMIT combined with ORDER BY
+ often will use an index because only a small portion of the table
+ is returned.
When using wild-card operators such as LIKE or
~, indexes can only be used if the beginning of the search
- is anchored to the start of the string. So, to use indexes,
- LIKE searches should not begin with %, and
- ~(regular expression searches) should start with
- ^.
+ is anchored to the start of the string. Therefore, to use indexes,
+ LIKE patterns must not start with %, and
+ ~(regular expression) patterns must start with ^.
4.9) How do I see how the query optimizer is
evaluating my query?
@@ -1091,13 +1094,14 @@ BYTEA bytea variable-length byte array (null-byte safe)
No. Currval() returns the current value assigned by your
backend, not by all users.
- 4.15.4) Why aren't my sequence numbers reused
- on transaction abort? Why are there gaps in the numbering of my
- sequence/SERIAL column?
+ 4.15.4) Why aren't my sequence numbers
+ reused on transaction abort? Why are there gaps in the numbering of
+ my sequence/SERIAL column?
To improve concurrency, sequence values are given out to running
transactions as needed and are not locked until the transaction
- completes. This causes gaps in numbering from aborted transactions.
+ completes. This causes gaps in numbering from aborted
+ transactions.
4.16) What is an OID? What is
a TID?