On Thu, Jan 27, 2011 at 2:18 PM, Igor Neyman <ineyman@xxxxxxxxxxxxxx> wrote: > >> -----Original Message----- >> From: Scott Marlowe [mailto:scott.marlowe@xxxxxxxxx] >> Sent: Thursday, January 27, 2011 4:16 PM >> To: Igor Neyman >> Cc: Mladen Gogala; Tom Lane; David Wilson; Kenneth Marshall; >> pgsql-performance@xxxxxxxxxxxxxx >> Subject: Re: Postgres 9.0 has a bias against indexes >> >> On Thu, Jan 27, 2011 at 2:12 PM, Igor Neyman >> <ineyman@xxxxxxxxxxxxxx> wrote: >> > >> > >> >> -----Original Message----- >> >> From: Scott Marlowe [mailto:scott.marlowe@xxxxxxxxx] >> >> Sent: Thursday, January 27, 2011 3:59 PM >> >> To: Mladen Gogala >> >> Cc: Igor Neyman; Tom Lane; David Wilson; Kenneth Marshall; >> >> pgsql-performance@xxxxxxxxxxxxxx >> >> Subject: Re: Postgres 9.0 has a bias against indexes >> >> >> >> On Thu, Jan 27, 2011 at 1:44 PM, Mladen Gogala >> >> <mladen.gogala@xxxxxxxxxxx> wrote: >> >> > On 1/27/2011 3:37 PM, Scott Marlowe wrote: >> >> >> >> >> >> On Thu, Jan 27, 2011 at 1:31 PM, Mladen Gogala >> >> >> <mladen.gogala@xxxxxxxxxxx> wrote: >> >> >>> >> >> >>> There is INDEX UNIQUE SCAN PK_EMP. Oracle will use an index. >> >> >> >> >> >> That's because Oracle has covering indexes. >> >> >> >> >> > I am not sure what you mean by "covering indexes" but I >> >> hope that for >> >> > the larger table I have in mind, indexes will be used. >> >> For a small >> >> > table like >> >> >> >> In Oracle you can hit JUST the index to get the data you need (and >> >> maybe rollback logs, which are generally pretty small) >> >> >> >> In Pgsql, once you hit the index you must then hit the actual data >> >> store to get the right version of your tuple. So, index >> access in pg >> >> is more expensive than in Oracle. However, updates are cheaper. >> >> Always a trade off >> >> >> >> >> > >> > Scott, >> > What you describe here isn't about "covering indexes" - >> it's about different ways implementing MVCC in Oracle and PG. >> >> It is about covering indexes AND it's about the difference in >> how MVCC is implemented in both databases. >> >> > > Well, Mladen's query doesn't involve covering indexes. On Oracle? Then how can it get the values it needs without having to hit the data store? -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
