Обсуждение: TEXT field and Postgresql Perfomance
Do large TEXT or VARCHAR entries in postgresql cause any performance degradation when a query is being executed to search for data in a table where the TEXT/VARCHAR fields aren't being searched themselves? Since, according to the postgresql docs, theirs no performance difference between VARCHAR and TEXT, I'm assuming VARCHAR is identical to TEXT entries with a restriction set on the length. And since TEXT can be of any possible size, then they must be stored independently of the rest of the table which is probably all stored in a fixed size rows since all or nearly all of the other types have a specific size including CHAR. Therefore TEXT entries must be in some other hash table that only needs to be looked up when that column is referenced. If this is the case then all other row data will need to be read in for an unindexed query, but the TEXT columns will only be read if their being searched though or queried. And if they're only being queried, then only the rows that matched will need the TEXT columns read in which should have minimal impact on performance even if they contain kilobytes of information. -- I sense much NT in you. NT leads to Bluescreen. Bluescreen leads to downtime. Downtime leads to suffering. NT is the path to the darkside. Powerful Unix is. Public Key: ftp://ftp.tallye.com/pub/lorenl_pubkey.asc Fingerprint: B3B9 D669 69C9 09EC 1BCD 835A FAF3 7A46 E4A3 280C
On Fri, Jan 07, 2005 at 19:36:47 -0800, "Loren M. Lang" <lorenl@alzatex.com> wrote: > Do large TEXT or VARCHAR entries in postgresql cause any performance > degradation when a query is being executed to search for data in a table > where the TEXT/VARCHAR fields aren't being searched themselves? Yes in that the data is more spread out because of the wider rows and that results in more disk blocks being looked at to get the desired data. > Since, according to the postgresql docs, theirs no performance > difference between VARCHAR and TEXT, I'm assuming VARCHAR is identical > to TEXT entries with a restriction set on the length. And since TEXT > can be of any possible size, then they must be stored independently of No. > the rest of the table which is probably all stored in a fixed size rows No, Postgres uses variable length records.
I guess my question that would follow is, when does it work best to start using BLOBs/CLOBs (I forget if pg has CLOBs) instead of textfields because your table is going to balloon in disk blocks if you have large amounts of data, and all fields you want to search on would have to be indexed, increasing insert time substantialy. Does it ever pay to use text and not CLOB unless your text is going to be short, in which case why not just varchar, leading to the thought that the text datatype is just bad? Alex Turner NetEconomist On Fri, 7 Jan 2005 22:03:23 -0600, Bruno Wolff III <bruno@wolff.to> wrote: > On Fri, Jan 07, 2005 at 19:36:47 -0800, > "Loren M. Lang" <lorenl@alzatex.com> wrote: > > Do large TEXT or VARCHAR entries in postgresql cause any performance > > degradation when a query is being executed to search for data in a table > > where the TEXT/VARCHAR fields aren't being searched themselves? > > Yes in that the data is more spread out because of the wider rows and that > results in more disk blocks being looked at to get the desired data. > > > Since, according to the postgresql docs, theirs no performance > > difference between VARCHAR and TEXT, I'm assuming VARCHAR is identical > > to TEXT entries with a restriction set on the length. And since TEXT > > can be of any possible size, then they must be stored independently of > > No. > > > the rest of the table which is probably all stored in a fixed size rows > > No, Postgres uses variable length records. > > ---------------------------(end of broadcast)--------------------------- > TIP 7: don't forget to increase your free space map settings >
On Fri, Jan 07, 2005 at 10:03:23PM -0600, Bruno Wolff III wrote: > On Fri, Jan 07, 2005 at 19:36:47 -0800, "Loren M. Lang" <lorenl@alzatex.com> wrote: > > > Since, according to the postgresql docs, theirs no performance > > difference between VARCHAR and TEXT, I'm assuming VARCHAR is identical > > to TEXT entries with a restriction set on the length. And since TEXT > > can be of any possible size, then they must be stored independently of > > No. > > > the rest of the table which is probably all stored in a fixed size rows > > No, Postgres uses variable length records. A discussion of TOAST and ALTER TABLE SET STORAGE might be appropriate here, but I'll defer that to somebody who understands such things better than me. -- Michael Fuhr http://www.fuhr.org/~mfuhr/
Bruno Wolff III <bruno@wolff.to> writes:
> On Fri, Jan 07, 2005 at 19:36:47 -0800,
> "Loren M. Lang" <lorenl@alzatex.com> wrote:
>> Do large TEXT or VARCHAR entries in postgresql cause any performance
>> degradation when a query is being executed to search for data in a table
>> where the TEXT/VARCHAR fields aren't being searched themselves?
> Yes in that the data is more spread out because of the wider rows and that
> results in more disk blocks being looked at to get the desired data.
You are overlooking the effects of TOAST. Fields wider than a kilobyte
or two will be pushed out-of-line and will thereby not impose a penalty
on queries that only access the other fields in the table.
(If Loren's notion of "large" is "a hundred bytes" then there may be a
measurable impact. If it's "a hundred K" then there won't be.)
regards, tom lane