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Bruce Momjian 95336f037d Add type conversion TODO.detail
2000-06-14 03:30:49 +00:00

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From pgsql-hackers-owner+M1833@hub.org Sat May 13 22:49:26 2000
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To: pgsql-hackers@postgresql.org
Subject: [HACKERS] Proposal for fixing numeric type-resolution issues
Date: Sat, 13 May 2000 22:40:38 -0400
Message-ID: <18340.958272038@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
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Status: ORr
We've got a collection of problems that are related to the parser's
inability to make good type-resolution choices for numeric constants.
In some cases you get a hard error; for example "NumericVar + 4.4"
yields
ERROR: Unable to identify an operator '+' for types 'numeric' and 'float8'
You will have to retype this query using an explicit cast
because "4.4" is initially typed as float8 and the system can't figure
out whether to use numeric or float8 addition. A more subtle problem
is that a query like "... WHERE Int2Var < 42" is unable to make use of
an index on the int2 column: 42 is resolved as int4, so the operator
is int24lt, which works but is not in the opclass of an int2 index.
Here is a proposal for fixing these problems. I think we could get this
done for 7.1 if people like it.
The basic problem is that there's not enough smarts in the type resolver
about the interrelationships of the numeric datatypes. All it has is
a concept of a most-preferred type within the category of numeric types.
(We are abusing the most-preferred-type mechanism, BTW, because both
FLOAT8 and NUMERIC claim to be the most-preferred type in the numeric
category! This is in fact why the resolver can't make a choice for
"numeric+float8".) We need more intelligence than that.
I propose that we set up a strictly-ordered hierarchy of numeric
datatypes, running from least preferred to most preferred:
int2, int4, int8, numeric, float4, float8.
Rather than simply considering coercions to the most-preferred type,
the type resolver should use the following rules:
1. No value will be down-converted (eg int4 to int2) except by an
explicit conversion.
2. If there is not an exact matching operator, numeric values will be
up-converted to the highest numeric datatype present among the operator
or function's arguments. For example, given "int2 + int8" we'd up-
convert the int2 to int8 and apply int8 addition.
The final piece of the puzzle is that the type initially assigned to
an undecorated numeric constant should be NUMERIC if it contains a
decimal point or exponent, and otherwise the smallest of int2, int4,
int8, NUMERIC that will represent it. This is a considerable change
from the current lexer behavior, where you get either int4 or float8.
For example, given "NumericVar + 4.4", the constant 4.4 will initially
be assigned type NUMERIC, we will resolve the operator as numeric plus,
and everything's fine. Given "Float8Var + 4.4", the constant is still
initially numeric, but will be up-converted to float8 so that float8
addition can be used. The end result is the same as in traditional
Postgres: you get float8 addition. Given "Int2Var < 42", the constant
is initially typed as int2, since it fits, and we end up selecting
int2lt, thereby allowing use of an int2 index. (On the other hand,
given "Int2Var < 100000", we'd end up using int4lt, which is correct
to avoid overflow.)
A couple of crucial subtleties here:
1. We are assuming that the parser or optimizer will constant-fold
any conversion functions that are introduced. Thus, in the
"Float8Var + 4.4" case, the 4.4 is represented as a float8 4.4 by the
time execution begins, so there's no performance loss.
2. We cannot lose precision by initially representing a constant as
numeric and later converting it to float. Nor can we exceed NUMERIC's
range (the default 1000-digit limit is more than the range of IEEE
float8 data). It would not work as well to start out by representing
a constant as float and then converting it to numeric.
Presently, the pg_proc and pg_operator tables contain a pretty fair
collection of cross-datatype numeric operators, such as int24lt,
float48pl, etc. We could perhaps leave these in, but I believe that
it is better to remove them. For example, if int42lt is left in place,
then it would capture cases like "Int4Var < 42", whereas we need that
to be translated to int4lt so that an int4 index can be used. Removing
these operators will eliminate some code bloat and system-catalog bloat
to boot.
As far as I can tell, this proposal is almost compatible with the rules
given in SQL92: in particular, SQL92 specifies that an operator having
both "approximate numeric" (float) and "exact numeric" (int or numeric)
inputs should deliver an approximate-numeric result. I propose
deviating from SQL92 in a single respect: SQL92 specifies that a
constant containing an exponent (eg 1.2E34) is approximate numeric,
which implies that the result of an operator using it is approximate
even if the other operand is exact. I believe it's better to treat
such a constant as exact (ie, type NUMERIC) and only convert it to
float if the other operand is float. Without doing that, an assignment
like
UPDATE tab SET NumericVar = 1.234567890123456789012345E34;
will not work as desired because the constant will be prematurely
coerced to float, causing precision loss.
Comments?
regards, tom lane
From tgl@sss.pgh.pa.us Sun May 14 17:30:56 2000
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To: Bruce Momjian <pgman@candle.pha.pa.us>
cc: PostgreSQL-development <pgsql-hackers@postgreSQL.org>
Subject: Re: [HACKERS] type conversion discussion
In-reply-to: <200005141950.PAA04636@candle.pha.pa.us>
References: <200005141950.PAA04636@candle.pha.pa.us>
Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
message dated "Sun, 14 May 2000 15:50:20 -0400"
Date: Sun, 14 May 2000 17:29:30 -0400
Message-ID: <20911.958339770@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
Status: OR
Bruce Momjian <pgman@candle.pha.pa.us> writes:
> As some point, it seems we need to get all the PostgreSQL minds together
> to discuss type conversion issues. These problems continue to come up
> from release to release. We are getting better, but it seems a full
> discussion could help solidify our strategy.
OK, here are a few things that bug me about the current type-resolution
code:
1. Poor choice of type to attribute to numeric literals. (A possible
solution is sketched in my earlier message, but do we need similar
mechanisms for other type categories?)
2. Tensions between treating string literals as "unknown" type and
as "text" type, per this thread so far.
3. IS_BINARY_COMPATIBLE seems like a bogus concept. Do we really want a
fully symmetrical ring of types in each group? I'd prefer to see a
one-way equivalence, which allows eg. OID to be silently converted
to INT4, but *not* vice versa (except perhaps by specific user cast).
This'd be more like a traditional "is-a" or inheritance relationship
between datatypes, which has well-understood semantics.
4. I'm also concerned that the behavior of IS_BINARY_COMPATIBLE isn't
very predictable because it will happily go either way. For example,
if I do
select * from pg_class where oid = 1234;
it's unclear whether I will get an oideq or an int4eq operator ---
and that's a rather critical point since only one of them can exploit
an index on the oid column. Currently, there is some klugery in the
planner that works around this by overriding the parser's choice of
operator to substitute one that is compatible with an available index.
That's a pretty ugly solution ... I'm not sure I know a better one,
but as long as we're discussing type resolution issues ...
5. Lack of extensibility. There's way too much knowledge hard-wired
into the parser about type categories, preferred types, binary
compatibility, etc. All of it falls down when faced with
user-defined datatypes. If we do something like I suggested with
a hardwired hierarchy of numeric datatypes, it'll get even worse.
All this stuff ought to be driven off fields in pg_type rather than
be hardwired into the code, so that the same concepts can be extended
to user-defined types.
I don't have worked-out proposals for any of these but the first,
but they've all been bothering me for a while.
regards, tom lane
From tgl@sss.pgh.pa.us Sun May 14 21:02:31 2000
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To: Bruce Momjian <pgman@candle.pha.pa.us>
cc: PostgreSQL-development <pgsql-hackers@postgreSQL.org>
Subject: Re: [HACKERS] type conversion discussion
In-reply-to: <20911.958339770@sss.pgh.pa.us>
References: <200005141950.PAA04636@candle.pha.pa.us> <20911.958339770@sss.pgh.pa.us>
Comments: In-reply-to Tom Lane <tgl@sss.pgh.pa.us>
message dated "Sun, 14 May 2000 17:29:30 -0400"
Date: Sun, 14 May 2000 21:03:17 -0400
Message-ID: <21258.958352597@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
Status: OR
Here are the results of some further thoughts about type-conversion
issues. This is not a complete proposal yet, but a sketch of an
approach that might solve several of the gripes in my previous proposal.
While thinking about this, I realized that my numeric-types proposal
of yesterday would break at least a few cases that work nicely now.
For example, I frequently do things like
select * from pg_class where oid = 1234;
whilst poking around in system tables and querytree dumps. If that
constant is initially resolved as int2, as I suggested yesterday,
then we have "oid = int2" for which there is no operator. To succeed
we must decide to promote the constant to int4 --- but with no int4
visible among the operands of the "=", it will not work to just "promote
numerics to the highest type seen in the operands" as I suggested
yesterday. So there has to be some more interaction in there.
Anyway, I was complaining about the looseness of the concept of
binary-compatible types and the fact that the parser's type conversion
knowledge is mostly hardwired. These might be resolved by generalizing
the numeric type hierarchy idea into a "type promotion lattice", which
would work like this:
* Add a "typpromote" column to pg_type, which contains either zero or
the OID of another type that the parser is allowed to promote this
type to when searching for usable functions/operators. For example,
my numeric-types hierarchy of yesterday would be expressed by making
int2 promote to int4, int4 to int8, int8 to numeric, numeric to
float4, and float4 to float8. The promotion idea also replaces the
current concept of binary-compatible types: for example, OID would
link to int4 and varchar would link to text (but not vice versa!).
* Also add a "typpromotebin" boolean column to pg_type, which contains
't' if the type conversion indicated by typpromote is "free", ie,
no conversion function need be executed before regarding a value as
belonging to the promoted type. This distinguishes binary-compatible
from non-binary-compatible cases. If "typpromotebin" is 'f' and the
parser decides it needs to apply the conversion, then it has to look
up the appropriate conversion function in pg_proc. (More about this
below.)
Now, if the parser fails to find an exact match for a given function
or operator name and the exact set of input data types, it proceeds by
chasing up the promotion chains for the input data types and trying to
locate a set of types for which there is a matching function/operator.
If there are multiple possibilities, we choose the one which is the
"least promoted" by some yet-to-be-determined metric. (This metric
would probably favor "free" conversions over non-free ones, but other
than that I'm not quite sure how it should work. The metric would
replace a whole bunch of ad-hoc heuristics that are currently applied
in the type resolver, so even if it seems rather ad-hoc it'd still be
cleaner than what we have ;-).)
In a situation like the "oid = int2" example above, this mechanism would
presumably settle on "int4 = int4" as being the least-promoted
equivalent operator. (It could not find "oid = oid" since there is
no promotion path from int2 to oid.) That looks bad since it isn't
compatible with an oidops index --- but I have a solution for that!
I don't think we need the oid opclass at all; why shouldn't indexes
on oid be expressed as int4 indexes to begin with? In general, if
two types are considered binary-equivalent under the old scheme, then
the one that is considered the subtype probably shouldn't have separate
index operators under this new scheme. Instead it should just rely on
the index operators of the promoted type.
The point of the proposed typpromotebin field is to save a pg_proc
lookup when trying to determine whether a particular promotion is "free"
or not. We could save even more lookups if we didn't store the boolean
but instead the actual OID of the conversion function, or zero if the
promotion is "free". The trouble with that is that it creates a
circularity problem when trying to define a new user type --- you can't
define the conversion function if its input type doesn't exist yet.
In any case, we want the parser to do a function lookup if we've
advanced more than one step in the promotion hierarchy: if we've decided
to promote int4 to float8 (which will be a four-step chain through int8,
numeric, float4) we sure want the thing to use a direct int4tofloat8
conversion function if available, not a chain of four conversion
functions. So on balance I think we want to look in pg_proc once we've
decided which conversion to perform. The only reason for having
typpromotebin is that the promotion metric will want to know which
conversions are free, and we don't want to have to do a lookup in
pg_proc for each alternative we consider, only the ones that are finally
selected to be used.
I can think of at least one special case that still isn't cleanly
handled under this scheme, and that is bpchar vs. varchar comparison.
Currently, we have
regression=# select 'a'::bpchar = 'a '::bpchar;
?column?
----------
t
(1 row)
This is correct since trailing blanks are insignificant in bpchar land,
so the two values should be considered equal. If we try
regression=# select 'a'::bpchar = 'a '::varchar;
ERROR: Unable to identify an operator '=' for types 'bpchar' and 'varchar'
You will have to retype this query using an explicit cast
which is pretty bogus but at least it saves the system from making some
random choice about whether bpchar or varchar comparison rules apply.
On the other hand,
regression=# select 'a'::bpchar = 'a '::text;
?column?
----------
f
(1 row)
Here the bpchar value has been promoted to text and then text comparison
(where trailing blanks *are* significant) is applied. I'm not sure that
we can really justify doing this in this case when we reject the bpchar
vs varchar case, but maybe someone wants to argue that that's correct.
The natural setup in my type-promotion scheme would be that both bpchar
and varchar link to 'text' as their promoted type. If we do nothing
special then text-style comparison would be used in a bpchar vs varchar
comparison, which is arguably wrong.
One way to deal with this without introducing kluges into the type
resolver is to provide a full set of bpchar vs text and text vs bpchar
operators, and make sure that the promotion metric is such that these
will be used in place of text vs text operators if they apply (which
should hold, I think, for any reasonable metric). This is probably
the only way to get the "right" behavior in any case --- I think that
the "right" behavior for such comparisons is to strip trailing blanks
from the bpchar side but not the text/varchar side. (I haven't checked
to see if SQL92 agrees, though.)
Another issue is how to fit resolution of "unknown" literals into this
scheme. We could probably continue to handle them more or less as we
do now, but they might complicate the promotion metric.
I am not clear yet on whether we'd still need the concept of "type
categories" as they presently exist in the resolver. It's possible
that we wouldn't, which would be a nice simplification. (If we do
still need them, we should have a column in pg_type that defines the
category of a type, instead of hard-wiring category assignments.)
regards, tom lane
From e99re41@DoCS.UU.SE Mon May 15 07:39:03 2000
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From: Peter Eisentraut <e99re41@DoCS.UU.SE>
Reply-To: Peter Eisentraut <peter_e@gmx.net>
To: Tom Lane <tgl@sss.pgh.pa.us>
cc: Bruce Momjian <pgman@candle.pha.pa.us>,
PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] type conversion discussion
In-Reply-To: <20911.958339770@sss.pgh.pa.us>
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Status: OR
On Sun, 14 May 2000, Tom Lane wrote:
> 1. Poor choice of type to attribute to numeric literals. (A possible
> solution is sketched in my earlier message, but do we need similar
> mechanisms for other type categories?)
I think your plan looks good for the numerical land. (I'll ponder the oid
issues in a second.) For other type categories, perhaps not. Should a line
be promoted to a polygon so you can check if it contains a point? Or a
polygon to a box? Higher dimensions? :-)
> 2. Tensions between treating string literals as "unknown" type and
> as "text" type, per this thread so far.
Yes, while we're at it, let's look at this in detail. I claim that
something of the form 'xxx' should always be text (or char or whatever),
period. Let's consider the cases were this could potentially clash with
the current behaviour:
a) The target type is unambiguously clear, e.g., UPDATE ... SET. Then you
cast text to the target type. The effect is identical.
b) The target type is completely unspecified, e.g. CREATE TABLE AS SELECT
'xxx'; This will currently create an "unknown" column. It should arguably
create a "text" column.
Function argument resolution:
c) There is only one function and it has a "text" argument. No-brainer.
d) There is only one function and it has an argument other than text. Try
to cast text to that type. (This is what's done in general, isn't it?)
e) The function is overloaded for many types, amongst which is text. Then
call the text version. I believe this would currently fail, which I'd
consider a deficiency.
f) The function is overloaded for many types, none of which is text. In
that case you have to cast anyway, so you don't lose anything.
On thing to also keep in mind regarding required casting for (b) and (f)
is that SQL never allowed literals of "fancy" types (e.g., DATE) to have
undecorated 'yyyy-mm-dd' constants, you always have to say DATE
'yyyy-mm-dd'. What Postgres allows is a convencience where DATE would be
obvious or implied. In the end it's a win-win situation: you tell the
system what you want, and your code is clearer.
> 3. IS_BINARY_COMPATIBLE seems like a bogus concept.
At least it's bogus when used for types which are not actually binary
compatible, e.g. int4 and oid. The result of the current implementation is
that you can perfectly happily insert and retrieve negative numbers from
oid fields.
I'm not so sure about the value of this particular equivalency anyway.
AFAICS the only functions that make sense for oids are comparisons (incl.
min, max), adding integers to them, subtracting one oid from another.
Silent mangling with int4 means that you can multiply them, square them,
add floating point numbers to them (doesn't really work in practice
though), all things that have no business with oids.
I'd say define the operators that are useful for oids explicitly for oids
and require casts for all others, so the users know what they're doing.
The fact that an oid is also a number should be an implementation detail.
In my mind oids are like pointers in C. Indiscriminate mangling of
pointers and integers in C has long been dismissed as questionable coding.
Of course I'd be very willing to consider counterexamples to these
theories ...
--
Peter Eisentraut Sernanders v<>g 10:115
peter_e@gmx.net 75262 Uppsala
http://yi.org/peter-e/ Sweden
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