I have the following table in postgresql:
database=# \d dic
Table "public.dic"
Column | Type | Modifiers
-------------+-------------------------+-----------
id | bigint |
stringvalue | character varying(2712) |
database=# create index idStringvalue on dic(id,stringvalue);
ERROR: index row size 2728 exceeds maximum 2712 for index "idstringvalue"
HINT: Values larger than 1/3 of a buffer page cannot be indexed.
Consider a function index of an MD5 hash of the value, or use full text indexing.
I dont know why is the error is coming when the size of stringvalue is 2712.
I want to truncate all the stringvalue's in dic which cause the above error. However, I am not getting how to do so. Can someone please help me with this?
I am am even fine with deleting the rows which cause this error. Is there some way by which I may do so?
Your column probably contains multibyte data: whereas the varchar(2712) deals with that just fine, it kind of makes sense that the indexing algorithm would be computing the c-string length, since memory considerations is what the latter is worrying about.
Theoretically, you can't go wrong by dividing the limit by four, i.e. use an unbounded varchar for the column, and index the first 600 characters or so, e.g.:
create index on dic((left(stringvalue, 600)));
This does raise the question of whether you actually need to index anything this large, though, since the value of doing so primarily lies in sorting. Postgres (correctly) suggests that you use an md5 of the value (if you're only interested in strict equality) or full text search (if you're interested in fuzzy matching).
Related
Should I define a column type from actual length to nth power of 2?
The first case, I have a table column store no more than 7 charactors,
will I use NVARCHAR(8)? since there maybe implicit convert inside Sql
server, allocate 8 space and truncate automatic(heard some where).
If not, NCHAR(7)/NCHAR(8), which should be(assume the fixed length is 7)
Any performance differ on about this 2 cases?
You should use the actual length of the string. Now, if you know that the value will always be exactly 7 characters, then use CHAR(7) rather than VARCHAR(7).
The reason you see powers-of-2 is for columns that have an indeterminate length -- a name or description that may not be fixed. In most databases, you need to put in some maximum length for the varchar(). For historical reasons, powers-of-2 get used for such things, because of the binary nature of the underlying CPUs.
Although I almost always use powers-of-2 in these situations, I can think of no real performance differences. There is one. . . in some databases the actual length of a varchar(255) is stored using 1 byte whereas a varchar(256) uses 2 bytes. That is a pretty minor difference -- even when multiplied over millions of rows.
I'm trying to use Google Cloud Datastore to store METAR observations (airport weather observations) but I am experiencing what I think is exploding indexes. My index for station_id (which is a 4 character string) is 20 times larger than the actual data itself. The database will increase by roughly 250 000 entities per day, so index size will become an issue.
Table
- observation_time (Date / Time) - indexed
- raw_text (String) (which is ~200 characters) - unindexed
- station_id (String) (which is always 4 characters) - indexed
Composite index:
- station_id (ASC), observation_time (ASC)
Query
The only query I will ever run is:
query.add_filter('station_id', '=', station_icao)
query.add_filter('observation_time', '>=', before)
query.add_filter('observation_time', '<=', after)
where before and after are datetime values
Index sizes
name type count size index size
observation_time Date/Time 1,096,184 26.14MB 313.62MB
station_id String 1,096,184 16.73MB 294.8MB
Datastore reports:
Resource Count Size
Entities 1,096,184 244.62MB
Built-in-indexes 5,488,986 740.63MB
Composite indexes 1,096,184 137.99MB
Help
I guess my first question is: What am I missing? I assume I'm doing something un-optimized, but I can't figure out what. Query time is not an immediate issue here, as long as lookups stays below ~2s.
Can I simply remove the built-in indexes, will the composite continue to work?
I've read up on Google and StackOverflow but can't seem to wrap my head around this. The reason I simply don't try to remove all built-in indexes is that it takes quite some time to download/un-index/put all the data afterwards I need to way 48hours for the dashboard summary to update - ie it will take me days before I get a result.
As +Jeffrey Rennie pointed out, "Exploding Indexes" is a very specific term that does not apply here.
You can see how storage size is calculate from our documentation here, so you can apply it to your example to see where the size adds up.
TL;DR: You can save space by using slightly more concise (but still readable!) property names. For example, observation_time to observation, etc
Key things to keep in mind:
To have a composite index, you need to have the individual properties indexed, so don't remove the built-ins or it'll stop working
Built-ins are indexed twice - once for ascending and once for descending
Kind names and property names are strings used in the index for each entity, so the longer they are the bigger the indexes
We are implementing a file upload and storage field in our DB2 database. Right now the file upload column is defined as BLOB(5242880) as follows:
CREATE TABLE MYLIB.MYTABLE (
REC_ID FOR COLUMN RECID DECIMAL(10, 0) GENERATED ALWAYS AS IDENTITY (
START WITH 1 INCREMENT BY 1
NO MINVALUE NO MAXVALUE
NO CYCLE NO ORDER
CACHE 20 )
,
[other fields snipped]
FORM_UPLOAD FOR COLUMN F00001 BLOB(5242880) DEFAULT NULL ,
CONSTRAINT MYLIB.MYCONSTRAINT PRIMARY KEY( REC_ID ) )
RCDFMT MYTABLER ;
Is this the correct way to do this? Should it be in its own table or defined a different way? I'm a little nervous that it's showing it as a five-megabyte column instead of a pointer to somwhere else, as SQL Server does (for example). Will we get into trouble defining it like this?
There's nothing wrong with storing the BLOB in a DB2 column, but if you prefer to store the pointer, look at DataLinks. http://pic.dhe.ibm.com/infocenter/iseries/v7r1m0/topic/sqlp/rbafyusoocap.htm
Unless you specify the ALLOCATE clause, the data itself is stored in the "variable length" aka "overflow" space of the table. Not the fixed length space where the rest of the row is.
So if you don't have ALLOCATE and the file is only 1MB, you only use 1MB of space to store it, not the 5MB max you've defined.
Note this means means the system has to do twice the I/O when accessing data from both areas.
Datalinks have the same I/O hit.
From a performance standpoint,
- Make sure you only read the BLOB if you need to.
- if 90% or more of the BLOBs are say < 1MB, you could improve performance at the cost of space by saying ALLOCATE(1048576) while still allowing for the full 5MB to be stored. The first 1MB would be in the row the last 4MB in the overflow.
Charles
So SQL server places a limit of 900 bytes on an index. I have columns that are NVARCHAR(1000) that I need to search on. Full text search of these columns is not required because search will always occur on the complete value or a prefix of the complete value. I will never need to search for terms that lie in the middle/end of the value.
The rows of the tables in question will never be updated predicated on this index, and the actual values that exceed 450 chars are outliers that will never be searched for.
Given the above, is there any reason not to ignore the warning:
The total size of an index or primary key cannot exceed 900 bytes
?
We shouldn't ignore the warning as any subsequent INSERT or UPDATE statement that specifies data values that generates a key value longer than 900 bytes will fail. Following link might help:
http://decipherinfosys.wordpress.com/2007/11/06/the-900-byte-index-limitation-in-sql-server/
let's say I have a column on my table defined the following:
"MyColumn" smallint NULL
Storing a value like 0, 1 or something else should need 2 bytes (1). But how much space is needed if I set "MyColumn" to NULL? Will it need 0 bytes?
Are there some additional needed bytes for administration purpose or such things for every column/row?
(1) http://www.postgresql.org/docs/9.0/interactive/datatype-numeric.html
Laramie is right about the bitmap and links to the right place in the manual. Yet, this is almost, but not quite correct:
So for any given row with one or more nulls, the size added to it
would be that of the bitmap(N bits for an N-column table, rounded up).
One has to factor in data alignment. The HeapTupleHeader (per row) is 23 bytes long, actual column data always starts at a multiple of MAXALIGN (typically 8 bytes). That leaves one byte of padding that can be utilized by the null bitmap. In effect NULL storage is absolutely free for tables up to 8 columns.
After that, another MAXALIGN (typically 8) bytes are allocated for the next MAXALIGN * 8(typically 64) columns. Etc. Always for the total number of user columns (all or nothing). But only if there is at least one actual NULL value in the row.
I ran extensive tests to verify all of that. More details:
Does not using NULL in PostgreSQL still use a NULL bitmap in the header?
Null columns are not stored. The row has a bitmap at the start and one bit per column that indicates which ones are null or non-null. The bitmap could be omitted if all columns are non-null in a row. So for any given row with one or more nulls, the size added to it would be that of the bitmap(N bits for an N-column table, rounded up).
More in depth discussion from the docs here
It should need 1 byte (0x00) however it's the structure of the table that makes up most of the space, adding this one value might change something (Like adding a row) which needs more space than the sum of the data in it.
Edit: Laramie seems to know more about null than me :)