怎么理解PostgreSQL的PGIndexProperties

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在PostgreSQL 9.6之后,PG提供了三个函数来判定Index AM/Index/Index Column是否具备某些属性,包括pg_indexam_has_property/pg_index_has_property/pg_index_column_has_property.

pg_indexam_has_property
test whether an index access method has a specified property

属性名称	说明
can_order	Does the access method support ASC, DESC and related keywords in CREATE INDEX?
can_unique	Does the access method support unique indexes?
can_multi_col	Does the access method support indexes with multiple columns?
can_exclude	Does the access method support exclusion constraints?
can_includev	Does the access method support the INCLUDE clause of CREATE INDEX?

下面是本机AM的查询结果,其中heap是堆AM/blackhole_am是先前介绍过的黑洞AM.

testdb=# select a.amname, p.name, pg_indexam_has_property(a.oid,p.name)
testdb-# from pg_am a,
testdb-#      unnest(array['can_order','can_unique','can_multi_col','can_exclude']) p(name)
testdb-# order by a.amname;
    amname    |     name      | pg_indexam_has_property 
--------------+---------------+-------------------------
 blackhole_am | can_unique    | 
 blackhole_am | can_exclude   | 
 blackhole_am | can_multi_col | 
 blackhole_am | can_order     | 
 brin         | can_order     | f
 brin         | can_exclude   | f
 brin         | can_multi_col | t
 brin         | can_unique    | f
 btree        | can_order     | t
 btree        | can_unique    | t
 btree        | can_multi_col | t
 btree        | can_exclude   | t
 gin          | can_unique    | f
 gin          | can_order     | f
 gin          | can_multi_col | t
 gin          | can_exclude   | f
 gist         | can_unique    | f
 gist         | can_multi_col | t
 gist         | can_exclude   | t
 gist         | can_order     | f
 hash         | can_order     | f
 hash         | can_unique    | f
 hash         | can_multi_col | f
 hash         | can_exclude   | t
 heap         | can_multi_col | 
 heap         | can_unique    | 
 heap         | can_order     | 
 heap         | can_exclude   | 
 spgist       | can_multi_col | f
 spgist       | can_exclude   | t
 spgist       | can_unique    | f
 spgist       | can_order     | f
(32 rows)

PostgreSQL根据上述属性判断在创建索引时指定的option,如Hash索引不能是唯一索引(hash         | can_unique    | f):

testdb=# create unique index idx_t_idx1_id on t_idx1 using hash(id);
psql: ERROR:  access method "hash" does not support unique indexes

pg_index_has_property
test whether an index has a specified property

属性名称	说明
clusterable	Can the index be used in a CLUSTER command?
index_scan	Does the index support plain (non-bitmap) scans?
bitmap_scan	Does the index support bitmap scans?
backward_scan	Can the scan direction be changed in mid-scan (to support FETCH BACKWARD on a cursor without needing materialization)?

创建hash索引,查询该索引的相关属性

testdb=# create index idx_t_idx1_id on t_idx1 using hash(id);
CREATE INDEX
testdb=# select p.name, pg_index_has_property('idx_t_idx1_id'::regclass,p.name)
testdb-# from unnest(array[
testdb(#        'clusterable','index_scan','bitmap_scan','backward_scan'
testdb(#      ]) p(name);
     name      | pg_index_has_property 
---------------+-----------------------
 clusterable   | f
 index_scan    | t
 bitmap_scan   | t
 backward_scan | t
(4 rows)

pg_index_column_has_property
test whether an index column has a specified property

属性名称	说明
asc	Does the column sort in ascending order on a forward scan?
desc	Does the column sort in descending order on a forward scan?
nulls_first	Does the column sort with nulls first on a forward scan?
nulls_last	Does the column sort with nulls last on a forward scan?
orderable	Does the column possess any defined sort ordering?
distance_orderable	Can the column be scanned in order by a “distance” operator, for example ORDER BY col <-> constant ?
returnable	Can the column value be returned by an index-only scan?
search_array	Does the column natively support col = ANY(array) searches?
search_nulls	Does the column support IS NULL and IS NOT NULL searches?

查询hash索引列的相关属性(全为f - false)

testdb=# select p.name,
testdb-#      pg_index_column_has_property('idx_t_idx1_id'::regclass,1,p.name)
testdb-# from unnest(array[
testdb(#        'asc','desc','nulls_first','nulls_last','orderable','distance_orderable',
testdb(#        'returnable','search_array','search_nulls'
testdb(#      ]) p(name);
        name        | pg_index_column_has_property 
--------------------+------------------------------
 asc                | f
 desc               | f
 nulls_first        | f
 nulls_last         | f
 orderable          | f
 distance_orderable | f
 returnable         | f
 search_array       | f
 search_nulls       | f
(9 rows)

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