The `ColumnType` struct is not trivially small. Also calling a
by-pointer method on a copy may cause it to escape on the heap. Avoid
making copies until necessary.

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These will be used to break up table.go in smaller pieces.

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…estutils

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Since it's super small and used only in one place.

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to EncodeDatumKeyAscending and EncodeDatumsKeyAscending, respectively.
Also, add comments and move them to column_type_encoding.go.

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This was not well defined; the UNION code that required it really has
particular semantics and it is thus best defined close to the
implementation of UNION.

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It is unneeded; we already have a perfectly useful `TypeName` map in
package `oid` which does the same.

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Prior to this patch, CockroachDB incorrectly placed the "input syntax"
of each SQL type in the column `data_type` of
`information_schema.columns`.

The input syntax is the one reported in SHOW COLUMNS, SHOW CREATE
TABLE and other places, and is suitable to reproduce the exact type of
at able.

In contrast, `information_schema.columns.data_type` is constrained by
compatibility with third party tools and PostgreSQL clients.  It must
report the name of the type like PostgreSQL does, which in turn is
constrained by the SQL standard. A text column must be reported as
"text" not "string"; a decimal column as "numeric" not "decimal", a
float8 column as "double precision" not "float8", and so on.

By reporting the wrong string in that column CockroachDB is confusing
ORMs, which subsequently decide that the current on-disk type is not
the one expected by the app and then initiate a schema change (ALTER
COLUMN SET TYPE).

This patch corrects this incompatibility by introducing logic that
produces the proper information schema names for column types. This is
expected to reduce ORM complaints about insufficient support for ALTER
COLUMN SET TYPE (but will be tested/evaluated separately).

Release note (bug fix): CockroachDB now populates the `data_type`
column of `information_schema.columns` like PostgreSQL for
compatibility with 3rd party tools and ORMs.

There are major problems in `cockroach dump` described separately in
cockroachdb#28948.

Part of this is `cockroach dump` trying to reverse-engineering a datum
type from the announced column type in `information_schema.columns`,
by parsing the type name. Prior to this patch, this parsing was
incomplete (it went out of sync with the SQL parser over time,
naturally as it was bound to do).

This entire approach is flawed, but this is no time to redesign
`cockroach dump`. Instead, this patch re-syncs the dump-specific type
parser with the general SQL parser.

Release note (bug fix): `cockroach dump` is now again better able to
operate across multiple CockroachDB versions.

The distinction between "TEXT" and "STRING" inside CockroachDB is
inconsequential. Remove it. The data type is already properly reported
in pg_catalog and information_schema as "text".

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This patch corrects two long-standing bugs in CockroachDB which were
confusing both 3rd party tools/ORMs and users:

- the SQL standard type called "CHAR" is supposed to have a default
  maximum width of 1 character.

  Prior to this patch, CockroachDB did not support this and instead
  set no default maximum width on CHAR. This would cause CockroachDB
  to fail validating the width of values inserted in tables, fail
  to constrain the width of values during casts to CHAR, and report
  incorrect information in introspection tables.

- PostgresSQL's dialect distinguishes the table column types TEXT,
  CHAR, VARCHAR and a special thing called `"char"` (this is a legacy
  PostgreSQL type which is, unfortunately, still used by some
  pg_catalog tables).

  Prior to this patch, CockroachDB would map all of these types into
  the same column type "STRING". In turn this caused them to show
  up as "text" in introspection.

  While this was not incorrect from the perspective of value
  handling (all these types are, indeed, handled with about the same
  value semantics in both PostgreSQL and CockroachDB), ORMs
  do pay attention to this distinction and become confused if they
  see a different type in introspection (e.g. "text") than what
  the application model requires (e.g. "char"). The result of this
  confusion was to trigger a schema change to adapt the type, which
  in turn fails due to missing features in ALTER COLUMN TYPE.

This patch corrects both problems by giving the appropriate default
width to CHAR and preserving the distinction between the various string
types in column descriptors and thus introspection.

Additionally, a width constraint check on collated string columns was
missing. This is now fixed too.

Tables and columns created prior to this patch are unaffected.

Release note (bug fix): CockroachDB now distinguishes "CHAR" and
"VARCHAR" as mandated by the SQL standard and PostgreSQL
compatibility. When a width is not specified (e.g. `CHAR(3)`), the
maximum width of `VARCHAR` remains unconstrained whereas the maximum
width for `CHAR` is now 1 character.

Release note (bug fix): CockroachDB now properly checks the width of
strings inserted in a collated string column with a specified width.

Release note (sql change): CockroachDB now preserves the distinction
between different column types for string values like in PostgreSQL,
for compatibility with 3rd party tools and ORMs.

The distinction between the type names "JSON" and "JSONB" is
inconsequential. The types are already reported properly in
introspection using the right alias. This patch removes this
distinction in code.

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Prior to this patch the canonical name for the timestamptz type in
CockroachDB was "TIMESTAMP WITH TIME ZONE" with spaces and all
glorious 24 characters.

This is unfortunate because this is a pretty common type, and is
moreover used to disambiguate the return type of `now()` and thus will
be emitted pretty often in distsql physical plans. Having such a long
type name is unnecessary and indeed adds 50%+ storage, communication
and parsing overhead for every occurrence of this type in
distsql plans.

This patch shortens the canonical CockroachDB name to "TIMESTAMPTZ".

The representation of the type in introspection tables (pg_catalog,
information_schema) is unaffected.

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It was not needed. This simplifies.

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Prior to this patch, CockroachDB maintained an unnecessary distinction
between "INT" and "INTEGER", between "BIGINT" and "INT8", etc.

This distinction is unnecessary but also costly, as we were paying the
price of a "name" attribute in coltypes.TInt, with a string comparison
and hash table lookup on every use of the type.

What really matters is that the type shows up properly in
introspection; this has already been ensured by various
OID-to-pgcatalog mappings and the recently introduced
`InformationSchemaTypeName()`.

Any distinction beyond that is unnecessary and can be dropped from the
implementation.

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