Обсуждение: casting between range types
At one point, the question of casting between range types came up. At first, this seemed like a fairly reasonable suggestion, but now I don't think I like the semantics. A normal cast changes between essentially equivalent values in different domains. For instance 3 as an int4 is equivalent to 3.0 as a numeric. However, if we take the simple approach with range types and cast the bounds, we end up with some weird situations. First, a range is really a set. So if we take '[1,10)'::int4range and cast that to numrange, we end up moving from a set of exactly 9 elements to a set of an infinite number of elements. Going the other way is probably worse. Sometimes casts are a bit "lossy" and I suppose we could write that off. But things get weirder when the total order is different (e.g. different text collations). Then you end up with a completely different set of values, which doesn't sound like a cast to me at all. So, I'm leaning toward just not providing any casts from one range type to another. Thoughts? RegardsJeff Davis
On 31.08.2011 09:14, Jeff Davis wrote: > At one point, the question of casting between range types came up. At > first, this seemed like a fairly reasonable suggestion, but now I don't > think I like the semantics. > > A normal cast changes between essentially equivalent values in different > domains. For instance 3 as an int4 is equivalent to 3.0 as a numeric. > > However, if we take the simple approach with range types and cast the > bounds, we end up with some weird situations. > > First, a range is really a set. So if we take '[1,10)'::int4range and > cast that to numrange, we end up moving from a set of exactly 9 elements > to a set of an infinite number of elements. Going the other way is > probably worse. > > Sometimes casts are a bit "lossy" and I suppose we could write that off. > > But things get weirder when the total order is different (e.g. different > text collations). Then you end up with a completely different set of > values, which doesn't sound like a cast to me at all. > > So, I'm leaning toward just not providing any casts from one range type > to another. Can you only provide casts that make sense, like between int4 and numeric range types, and leave out the ones that don't? -- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com
On Wed, 2011-08-31 at 09:20 +0300, Heikki Linnakangas wrote: > On 31.08.2011 09:14, Jeff Davis wrote: > > First, a range is really a set. So if we take '[1,10)'::int4range and > > cast that to numrange, we end up moving from a set of exactly 9 elements > > to a set of an infinite number of elements. Going the other way is > > probably worse. ... > Can you only provide casts that make sense, like between int4 and > numeric range types, and leave out the ones that don't? There are certainly some casts that make sense, like int4range->int8range. Do you think int4range->numrange also makes sense? Regards,Jeff Davis
On 31.08.2011 18:09, Jeff Davis wrote: > On Wed, 2011-08-31 at 09:20 +0300, Heikki Linnakangas wrote: >> On 31.08.2011 09:14, Jeff Davis wrote: >>> First, a range is really a set. So if we take '[1,10)'::int4range and >>> cast that to numrange, we end up moving from a set of exactly 9 elements >>> to a set of an infinite number of elements. Going the other way is >>> probably worse. > > ... > >> Can you only provide casts that make sense, like between int4 and >> numeric range types, and leave out the ones that don't? > > There are certainly some casts that make sense, like > int4range->int8range. Do you think int4range->numrange also makes sense? Not sure. It depends on whether you think of '[1,8]'::int4range as a finite set of the integers between 1 and 8, or as a continuous range from 1 to 8. I don't see harm in providing explicit casts like that, but I would be very conservative with implicit and assignment casts. -- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com
On Wed, Aug 31, 2011 at 11:36 AM, Heikki Linnakangas <heikki.linnakangas@enterprisedb.com> wrote: > On 31.08.2011 18:09, Jeff Davis wrote: >> On Wed, 2011-08-31 at 09:20 +0300, Heikki Linnakangas wrote: >>> >>> On 31.08.2011 09:14, Jeff Davis wrote: >>>> >>>> First, a range is really a set. So if we take '[1,10)'::int4range and >>>> cast that to numrange, we end up moving from a set of exactly 9 elements >>>> to a set of an infinite number of elements. Going the other way is >>>> probably worse. >> ... >> >>> Can you only provide casts that make sense, like between int4 and >>> numeric range types, and leave out the ones that don't? >> >> There are certainly some casts that make sense, like >> int4range->int8range. Do you think int4range->numrange also makes sense? > > Not sure. It depends on whether you think of '[1,8]'::int4range as a finite > set of the integers between 1 and 8, or as a continuous range from 1 to 8. I > don't see harm in providing explicit casts like that, but I would be very > conservative with implicit and assignment casts. +1 for that approach. It's really annoying when you can't explicitly cast between data types, and it might be that you just allow coercion via I/O functions since it's unlikely to be a performance-critical operation. But I can't see why you would want any implicit or assignment casts at all. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company