Thread: Using indexes and keys
I'm a bit confused about the use of indexes with postgresql.
I've a number of questions that are probably obvious when you know
the answer, but I can't find in the documentation.
1) It seems that I can only have primary keys in postgresql,
does this mean that keys are not the same as indexes ?
2) If I define a column as a primary key, does that mean that it will be
indexed, so I don't nee to create an index as well ?
2a) If so - If I define two or more columns as the primary key, will they
be indexed seperately, or do I need to index the 2nd and 3rd columns
seperately
3) If I create an index on a column (it is not unique and so can't be a key),
what do I need to query on - the index name or the column name ?
Thanks
JohnT
John,
> I'm a bit confused about the use of indexes with postgresql.
> I've a number of questions that are probably obvious when you know
> the answer, but I can't find in the documentation.
Actually, I like your questions. All of the below are obvious to
veteran DB programmers, but not to newbies. I'll put together a FAQ
based on your questions for Techdocs. Few of the answers below are
particular to Postgres; you will find them similar across all SQL
RDBMSs.
> 1) It seems that I can only have primary keys in postgresql,
> does this mean that keys are not the same as indexes ?
Correct, they are not the same. A "key" is a unique set of data that
allows you to uniquely identify a row. One of these Keys, chosen by
you, becomes a "Primary Key" by which the database system finds the
row for which you're looking. The other Keys, if any, are "candidate
keys." You may wish to put Constraints on these Keys to enforce their
uniqueness. Example:
Create Table staff (
staff_id SERIAL NOT NULL PRIMARY KEY,
staff_name VARCHAR (100) NOT NULL,
SSN VARCHAR (9) NOT NULL,
department VARCHAR (25) NOT NULL,
hire_date TIMESTAMP NOT NULL,
comments VARCHAR(200) NULL,
CONSTRAINT cs_SSN_unq UNIQUE(SSN),
CONSTRAINT cs_staff_combo_unq UNIQUE (staff_name, department,
hire_date)
);
In the table above, the integer index value staff_id is my Primary Key.
The SSN is a Candidate Key which I have chosen not to make my Primary
Key. Because of the uniqueness constraint, the combination
ofstaff_name, department, and hire_date is also a Candidate Key.
If I have two natural Candidate Keys, why did I choose to use a SERIAL
integer ID value? Convenience. SERIAL ID's are the easiest things to
work with as Primary Keys in most cases. Because the ID has no
purpose outside of providing a Primary Key, you don't have to worry
about it changing for an existing record, or getting re-used.
> 2) If I define a column as a primary key, does that mean that it will
> be
> indexed, so I don't nee to create an index as well ?
In Postgres and most other RDBMSs, yes. When you declare a Primary
Key, Postgres will tell you that it is creating both an index and a
uniqueness constraint on that column. While neither the index nor the
constraint are "part of" the key, Postgres (like most RDBMSs) needs
them to enforce the primary key.
> 2a) If so - If I define two or more columns as the primary key, will
> they
> be indexed seperately, or do I need to index the 2nd and 3rd
> columns
> seperately
They will be indexed together. So, if you need to do lookups on the
3rd column seperate from the 1st, you will need to build a seperate
index for that column alone.
CAUTION: From my 7 years of RDBMS programming experience, multi-column
primary keys are a huge headache and you will soon find yourself
re-building the table with an independant SERIAL primary key. If you
want to know why, just try doing a LEFT OUTER JOIN on two tables with
3-column primary keys. Bleah!
> 3) If I create an index on a column (it is not unique and so can't be
> a key),
> what do I need to query on - the index name or the column name ?
The column name. Your request (query) goes through a Query Planner,
which decides with good accuracy what the fastest way to find your
data is. Sometimes it uses your index, sometimes not, depending on
what you queried (indexes are not always faster). All you have to do
is run ANALYZE on your database periodically so that the Query Planner
has accurate current information to base its decisions on.
You would benefit from reading a few general RDBMS books. See the book
review page: http://techdocs.postgresql.org/bookreviews.php
Finally, the new version, 7.2, keeps statistics on the use your indexes
are seeing, which over time will help you improve your database by
dropping indexes that are not used and adding the ones you need.
-Josh Berkus
______AGLIO DATABASE SOLUTIONS___________________________
Josh Berkus
Complete information technology josh@agliodbs.com
and data management solutions (415) 565-7293
for law firms, small businesses fax 621-2533
and non-profit organizations. San Francisco
Josh,
I've taken your advise and ordered a book, but until that arrives,
one further question, if you don't mind:
On Friday 08 February 2002 17:49, Josh Berkus wrote:
>
> > 2a) If so - If I define two or more columns as the primary key, will
> > they
> > be indexed seperately, or do I need to index the 2nd and 3rd
> > columns
> > seperately
>
> They will be indexed together. So, if you need to do lookups on the
> 3rd column seperate from the 1st, you will need to build a seperate
> index for that column alone.
>
If I need to lookup on a combined key, how do I do that ?
The CREATE TABLE syntax doesn't seem to allow me to combine the columns into a single key name.
Additionally, if I combine two VARCHAR columns are the following keys unique ?
...
a VARCHAR(10),
b VARCHAR(10),
PRIMARY KEY (a,b),
...
a="abc", b="def"
a="ab", b="cdef"
If not, would I need to make a CHAR(10) rather than VARCHAR(10), or is there some other trick ?
Thanks
JohnT
John,
> If I need to lookup on a combined key, how do I do that ?
To SELECT or JOIN based on a multi-column key, you need to use WHERE
conditions for each column:
...FROM tablea JOIN tableb ON (tablea.c1 = tableb.c1 AND tablea.c2 =
tableb.c2)
If you want to make sure and use the multi-column index on that key,
make sure to keep the columns in the same order as they were in the
key declaration.
> The CREATE TABLE syntax doesn't seem to allow me to combine the
> columns into a single key name.
Why not?
CREATE TABLE tablea (
col1 INT4 NOT NULL,
col2 INT4 NOT NULL,
CONSTRAINT tablea_pk PRIMARY KEY (col1, col2)
);
> Additionally, if I combine two VARCHAR columns are the following keys
> unique ?
>
> ...
> a VARCHAR(10),
> b VARCHAR(10),
> PRIMARY KEY (a,b),
> ...
>
> a="abc", b="def"
>
> a="ab", b="cdef"
Yes, they are unique. Postgres does not concatinate fields to make a
multi-column key.
-Josh
P.S. For anyone just tuning into the thread, keep in mind that
multi-column keys are considerably more trouble than they're worth 75%
of the time.
Hi, About a week ago, I was out apt-get'ing things, and various dependencies decided postgresql needed to get upgraded. Fine, I don't use it for much yet, but had some data in it. During the install, I got a message saying I needed to do the dump/restore manually, and it went along on its merry way. Tonight, I went to do the dump. Debian has a shell script (supposedly) all set up to do this, but somehow I don't think it worked. If I run this custom script, I get some output to the screen at the end which is postgresql related, but looks like error messages from the log. The dump is 218 bytes long. If I try to run the 7.1 dumpall command manually, it tells me the postmaster isn't running. Which is correct, Debian has locked out the postmaster since the database dump needed to be done manually. Can this dump really be only 218 bytes long? Or should a person uninstall 7.2, reinstall 7.1, dump the data, uninstall 7.1, reinstall 7.2? Gord Matter Realisations http://www.materialisations.com/ Gordon Haverland, B.Sc. M.Eng. President 101 9504 182 St. NW Edmonton, AB, CA T5T 3A7 780/481-8019 ghaverla @ freenet.edmonton.ab.ca 780/993-1274 (cell)
On Wed, 2002-03-13 at 04:16, ghaverla@freenet.edmonton.ab.ca wrote: > Hi, > > About a week ago, I was out apt-get'ing things, and > various dependencies decided postgresql needed to get > upgraded. Fine, I don't use it for much yet, but > had some data in it. During the install, I got a > message saying I needed to do the dump/restore manually, > and it went along on its merry way. Tonight, I went to > do the dump. Debian has a shell script (supposedly) > all set up to do this, but somehow I don't think it worked. > If I run this custom script, I get some output to the screen > at the end which is postgresql related, but looks like > error messages from the log. The dump is 218 bytes long. > If I try to run the 7.1 dumpall command manually, it tells > me the postmaster isn't running. Which is correct, Debian > has locked out the postmaster since the database dump > needed to be done manually. You can start it. As root do: /etc/init.d/postgresql start and then connect with psql, but this will probably confirm that you have an empty datbase. > Can this dump really be only 218 bytes long? Or should > a person uninstall 7.2, reinstall 7.1, dump the data, > uninstall 7.1, reinstall 7.2? You have probably hit a bug in the automatic upgrade process of the Debian packaging. Your old database is stored in $PGDATA/../data.xx.save (where xx is the old version), but needs the 7.1 binaries to read it. These should be stored in /usr/lib/postgresql/dumpall/7.1 but there are sometimes problems along the way. This procedure is quite tricky, since it needs cooperation between the old and the new versions of the package. The latest release (7.2-5) has improved the procedure and may help; it is still in unstable. Please follow this procedure. In each of the directories in $PGDATA/.. check the contents of PG_VERSION, so as to be sure which is which. Make sure that the 7.1 directory is called $PGDATA. It would be wise to have a separate backup of this. Reinstall 7.1.3-7, check that the data is all present and correct and then retry the upgrade using 7.2-5. Then let me know how it goes. If possible, use script to capture the entire upgrade session. Since this is a Debian packaging issue, further discussion would be more appropriate in private email or on the Debian mailing lists. -- Oliver Elphick Oliver.Elphick@lfix.co.uk Isle of Wight http://www.lfix.co.uk/oliver GPG: 1024D/3E1D0C1C: CA12 09E0 E8D5 8870 5839 932A 614D 4C34 3E1D 0C1C "Who is like Thee among the gods, O Lord? Who is like Thee, majestic in holiness, awesome in praises, working wonders?" Exodus 15:11