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怎么理解PostgreSQL的PG Index Properties

发表于：2025-02-06 作者：千家信息网编辑

千家信息网最后更新 2025年02月06日，本篇内容介绍了"怎么理解PostgreSQL的PG Index Properties"的有关知识，在实际案例的操作过程中，不少人都会遇到这样的困境，接下来就让小编带领大家学习一下如何处理这些情况吧！希

千家信息网最后更新 2025年02月06日怎么理解PostgreSQL的PG Index Properties

本篇内容介绍了"怎么理解PostgreSQL的PG Index Properties"的有关知识，在实际案例的操作过程中，不少人都会遇到这样的困境，接下来就让小编带领大家学习一下如何处理这些情况吧！希望大家仔细阅读，能够学有所成！

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 INDEXtestdb=# 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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