public inbox for [email protected]help / color / mirror / Atom feed
What does "Table rewrite" mean? 5+ messages / 5 participants [nested] [flat]
* What does "Table rewrite" mean? @ 2018-01-19 16:03 PG Doc comments form <[email protected]> 0 siblings, 1 reply; 5+ messages in thread From: PG Doc comments form @ 2018-01-19 16:03 UTC (permalink / raw) To: [email protected]; +Cc: [email protected] The following documentation comment has been logged on the website: Page: https://www.postgresql.org/docs/9.5/static/sql-altertable.html Description: I see references to a "table rewrite" all over the place, but I cannot find one single definition on what that actually means. What does a table rewrite do? Does it drop and recreate the table? Everywhere I look people describe it with fear and trepedation as if it was some catastrophically dangerous operation to perform. What is it? ^ permalink raw reply [nested|flat] 5+ messages in thread
* Re: What does "Table rewrite" mean? @ 2018-01-19 16:33 Tom Lane <[email protected]> parent: PG Doc comments form <[email protected]> 0 siblings, 1 reply; 5+ messages in thread From: Tom Lane @ 2018-01-19 16:33 UTC (permalink / raw) To: [email protected]; +Cc: [email protected] =?utf-8?q?PG_Doc_comments_form?= <[email protected]> writes: > What does a table rewrite do? Does it drop and recreate the table? > Everywhere I look people describe it with fear and trepedation as if it was > some catastrophically dangerous operation to perform. What is it? It means reading the whole table and writing it out in some modified form (for instance, with some column transformed into a new datatype). It's not "dangerous" in any way ... but if you've got many GB of data in the table and you can't afford to have the table locked for a long time, then it's something to avoid. regards, tom lane ^ permalink raw reply [nested|flat] 5+ messages in thread
* Re: What does "Table rewrite" mean? @ 2018-01-20 10:18 Michael Paquier <[email protected]> parent: Tom Lane <[email protected]> 0 siblings, 1 reply; 5+ messages in thread From: Michael Paquier @ 2018-01-20 10:18 UTC (permalink / raw) To: Tom Lane <[email protected]>; +Cc: [email protected]; [email protected] On Fri, Jan 19, 2018 at 11:33:43AM -0500, Tom Lane wrote: > It means reading the whole table and writing it out in some modified > form (for instance, with some column transformed into a new datatype). > It's not "dangerous" in any way ... but if you've got many GB of data in > the table and you can't afford to have the table locked for a long time, > then it's something to avoid. Yeah that can be costly. Note that WAL corresponding to this data needs to be generated as well. -- Michael Attachments: [application/pgp-signature] signature.asc (833B, 2-signature.asc) download ^ permalink raw reply [nested|flat] 5+ messages in thread
* Re: What does "Table rewrite" mean? @ 2018-01-21 03:47 Alvaro Herrera <[email protected]> parent: Michael Paquier <[email protected]> 0 siblings, 1 reply; 5+ messages in thread From: Alvaro Herrera @ 2018-01-21 03:47 UTC (permalink / raw) To: Michael Paquier <[email protected]>; +Cc: Tom Lane <[email protected]>; [email protected]; [email protected] Michael Paquier wrote: > On Fri, Jan 19, 2018 at 11:33:43AM -0500, Tom Lane wrote: > > It means reading the whole table and writing it out in some modified > > form (for instance, with some column transformed into a new datatype). > > It's not "dangerous" in any way ... but if you've got many GB of data in > > the table and you can't afford to have the table locked for a long time, > > then it's something to avoid. > > Yeah that can be costly. Note that WAL corresponding to this data needs > to be generated as well. Maybe we need to document this somewhere, particularly now that we have a "table_rewrite" event item. -- Álvaro Herrera https://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services ^ permalink raw reply [nested|flat] 5+ messages in thread
* Re: What does "Table rewrite" mean? @ 2018-01-22 20:59 Ilsa Loving <[email protected]> parent: Alvaro Herrera <[email protected]> 0 siblings, 0 replies; 5+ messages in thread From: Ilsa Loving @ 2018-01-22 20:59 UTC (permalink / raw) To: Alvaro Herrera <[email protected]>; +Cc: Michael Paquier <[email protected]>; Tom Lane <[email protected]>; [email protected] That’s all that’s needed, really. It’s impossible to make an informed decision if there is no way for someone to know what a table rewrite actually does and how it does it. /*---------------------------------------------------------------------------------*/ float o=0.075,h=1.5,T,r,O,l,I;/* Ilsa Loving */int _,L=80,s=3200; int main() {for(;s%L||(h-=o,T=-2),s;4-(r=/* IT Manager */O*O)<(l=I*I)|++ _==L&& write(1,(--s%L?_<L?--_%6:6:7) /* The Jonah Group */+"Ilsa L.\n",1)&&(O=I=l=_=r=0, T+=o /2))O=I*2*O+h,I=l+T-r;} /* 416-304-0860x227 */ On 20 Jan 2018, at 22:47, Alvaro Herrera wrote: > Michael Paquier wrote: >> On Fri, Jan 19, 2018 at 11:33:43AM -0500, Tom Lane wrote: >>> It means reading the whole table and writing it out in some modified >>> form (for instance, with some column transformed into a new >>> datatype). >>> It's not "dangerous" in any way ... but if you've got many GB of >>> data in >>> the table and you can't afford to have the table locked for a long >>> time, >>> then it's something to avoid. >> >> Yeah that can be costly. Note that WAL corresponding to this data >> needs >> to be generated as well. > > Maybe we need to document this somewhere, particularly now that we > have > a "table_rewrite" event item. > > -- > Álvaro Herrera https://www.2ndQuadrant.com/ > PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services ^ permalink raw reply [nested|flat] 5+ messages in thread
end of thread, other threads:[~2018-01-22 20:59 UTC | newest] Thread overview: 5+ messages (download: mbox mbox.gz follow: Atom feed) -- links below jump to the message on this page -- 2018-01-19 16:03 What does "Table rewrite" mean? PG Doc comments form <[email protected]> 2018-01-19 16:33 ` Tom Lane <[email protected]> 2018-01-20 10:18 ` Michael Paquier <[email protected]> 2018-01-21 03:47 ` Alvaro Herrera <[email protected]> 2018-01-22 20:59 ` Ilsa Loving <[email protected]>
This inbox is served by agora; see mirroring instructions for how to clone and mirror all data and code used for this inbox