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Make collation-aware system catalog columns use "C" collation.

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Up to now we allowed text columns in system catalogs to use collation
"default", but that isn't really safe because it might mean something
different in template0 than it means in a database cloned from template0.
In particular, this could mean that cloned pg_statistic entries for such
columns weren't entirely valid, possibly leading to bogus planner
estimates, though (probably) not any outright failures.

In the wake of commit 5e0928005, a better solution is available: if we
label such columns with "C" collation, then their pg_statistic entries
will also use that collation and hence will be valid independently of
the database collation.

This also provides a cleaner solution for indexes on such columns than
the hack added by commit 0b28ea79c: the indexes will naturally inherit
"C" collation and don't have to be forced to use text_pattern_ops.

Also, with the planned improvement of type "name" to be collation-aware,
this policy will apply cleanly to both text and name columns.

Because of the pg_statistic angle, we should also apply this policy
to the tables in information_schema.  This patch does that by adjusting
information_schema's textual domain types to specify "C" collation.
That has the user-visible effect that order-sensitive comparisons to
textual information_schema view columns will now use "C" collation
by default.  The SQL standard says that the collation of those view
columns is implementation-defined, so I think this is legal per spec.
At some point this might allow for translation of such comparisons
into indexable conditions on the underlying "name" columns, although
additional work will be needed before that can happen.

Discussion: https://postgr.es/m/19346.1544895309@sss.pgh.pa.us
This commit is contained in:
Tom Lane
2018-12-18 12:48:15 -05:00
parent b2d9e17768
commit 6b0faf7236
9 changed files with 60 additions and 32 deletions
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13
src/backend/utils/cache/catcache.c vendored
View File

@ -22,6 +22,7 @@
#include "access/tuptoaster.h"
#include "access/valid.h"
#include "access/xact.h"
#include "catalog/pg_collation.h"
#include "catalog/pg_operator.h"
#include "catalog/pg_type.h"
#include "miscadmin.h"
@ -1014,8 +1015,8 @@ CatalogCacheInitializeCache(CatCache *cache)
/* Fill in sk_strategy as well --- always standard equality */
cache->cc_skey[i].sk_strategy = BTEqualStrategyNumber;
cache->cc_skey[i].sk_subtype = InvalidOid;
/* Currently, there are no catcaches on collation-aware data types */
cache->cc_skey[i].sk_collation = InvalidOid;
/* If a catcache key requires a collation, it must be C collation */
cache->cc_skey[i].sk_collation = C_COLLATION_OID;
CACHE4_elog(DEBUG2, "CatalogCacheInitializeCache %s %d %p",
cache->cc_relname,
@ -1961,10 +1962,10 @@ CatCacheCopyKeys(TupleDesc tupdesc, int nkeys, int *attnos,
NameData srcname;
/*
* Must be careful in case the caller passed a C string where a
* NAME is wanted: convert the given argument to a correctly
* padded NAME. Otherwise the memcpy() done by datumCopy() could
* fall off the end of memory.
* Must be careful in case the caller passed a C string where a NAME
* is wanted: convert the given argument to a correctly padded NAME.
* Otherwise the memcpy() done by datumCopy() could fall off the end
* of memory.
*/
if (att->atttypid == NAMEOID)
{