私のテーブルleadにはインデックスがあります：
\d lead
...
Indexes:
"lead_pkey" PRIMARY KEY, btree (id)
"lead_account__c" btree (account__c)
...
"lead_email" btree (email)
"lead_id_prefix" btree (id text_pattern_ops)
PG (9.1) がこの単純な等値選択にインデックスを使用しないのはなぜですか? 電子メールはほとんどすべてが一意です....
db=> explain select * from lead where email = 'blah';
QUERY PLAN
------------------------------------------------------------
Seq Scan on lead (cost=0.00..319599.38 rows=1 width=5108)
Filter: (email = 'blah'::text)
(2 rows)
他のインデックス ヒット クエリは問題ないようです (ただし、これが pkey インデックスを使用しない理由はわかりません)。
db=> explain select * from lead where id = '';
QUERY PLAN
------------------------------------------------------------------------------
Index Scan using lead_id_prefix on lead (cost=0.00..8.57 rows=1 width=5108)
Index Cond: (id = ''::text)
(2 rows)
db=> explain select * from lead where account__c = '';
QUERY PLAN
----------------------------------------------------------------------------------
Index Scan using lead_account__c on lead (cost=0.00..201.05 rows=49 width=5108)
Index Cond: (account__c = ''::text)
(2 rows)
最初は、 の明確な値が十分でないことが原因であると考えましたemail。たとえば、テーブルの大部分について統計が主張している場合はemail、blahseq スキャンの方が高速です。しかし、そうではありません。
db=> select count(*), count(distinct email) from lead;
count | count
--------+--------
749148 | 733416
(1 row)
seq スキャンを強制的にオフにしても、プランナーは他に選択肢がないかのように動作します。
db=> set enable_seqscan = off;
SET
db=> show enable_seqscan;
enable_seqscan
----------------
off
(1 row)
db=> explain select * from lead where email = 'foo@blah.com';
QUERY PLAN
---------------------------------------------------------------------------
Seq Scan on lead (cost=10000000000.00..10000319599.38 rows=1 width=5108)
Filter: (email = 'foo@blah.com'::text)
(2 rows)
また試しましたEXPLAIN ANALYZE：
db=> explain analyze select * from lead where email = 'foo@blah.com';
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------
Seq Scan on lead (cost=10000000000.00..10000319732.76 rows=1 width=5102) (actual time=77845.244..77845.244 rows=0 loops=1)
Filter: (email = 'foo@blah.com'::text)
Total runtime: 77857.215 ms
(3 rows)
出力は次のとおりです (申し訳ありませんが、列名を隠して、SO の制限に収まるようにトリミングする必要があります。http :\d //pastebin.com/ve3gzJpY でトリミングされていないバージョンを参照してください)。
Table "lead"
Column | Type | Modifiers
--------------------------------------------+-----------------------------+-----------
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX | real |
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX | text |
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX | boolean |
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX | text |
...
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX | text |
email | text |
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX | boolean |
...
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX | text |
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX | text |
account__c | text |
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX | text |
...
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX | text |
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX | text |
id | text | not null
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX | real |
...
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX | timestamp without time zone |
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX | real |
Indexes:
"lead_pkey" PRIMARY KEY, btree (id)
"lead_account__c" btree (account__c)
"lead_XXXXXXXXXXXXXXXXXXXXXX" btree (XXXXXXXXXXXXXXXXXXXXXX)
"lead_XXXXXXXXXXXXXXXXXXXXXX" btree (XXXXXXXXXXXXXXXXXXXXXX)
"lead_XXXXXXXXXXXXXXXXXXXXXX" btree (XXXXXXXXXXXXXXXXXXXXXX)
"lead_email" btree (email)
"lead_id_prefix" btree (id text_pattern_ops)
これは次のとおりです（再現性に役立つ場合に備えて、一意の列名も付けて、http://pastebin.com/ve3gzJpYpg_dump --schema-only -t leadでクロップされていないものを再度参照してください）：
--
-- PostgreSQL database dump
--
SET statement_timeout = 0;
SET client_encoding = 'UTF8';
SET standard_conforming_strings = on;
SET check_function_bodies = false;
SET client_min_messages = warning;
SET default_tablespace = '';
SET default_with_oids = false;
--
-- Name: lead; Type: TABLE; Schema: public; Owner: pod; Tablespace:
--
CREATE TABLE lead (
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX real,
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX text,
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX boolean,
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX text,
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX text,
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX date,
...
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX text,
account__c text,
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX text,
...
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX text,
id text NOT NULL,
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX real,
...
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX timestamp without time zone,
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX real
);
ALTER TABLE lead OWNER TO pod;
--
-- Name: lead_pkey; Type: CONSTRAINT; Schema: public; Owner: pod; Tablespace:
--
ALTER TABLE ONLY lead
ADD CONSTRAINT lead_pkey PRIMARY KEY (id);
--
-- Name: lead_account__c; Type: INDEX; Schema: public; Owner: pod; Tablespace:
--
CREATE INDEX lead_account__c ON lead USING btree (account__c);
--
-- Name: lead_XXXXXXXXXXXXXXXXXXXX; Type: INDEX; Schema: public; Owner: pod; Tablespace:
--
CREATE INDEX lead_XXXXXXXXXXXXXXXXXXXX ON lead USING btree (XXXXXXXXXXXXXXXXXXXX);
--
-- Name: lead_XXXXXXXXXXXXXXXXXXXX; Type: INDEX; Schema: public; Owner: pod; Tablespace:
--
CREATE INDEX lead_XXXXXXXXXXXXXXXXXXXX ON lead USING btree (XXXXXXXXXXXXXXXXXXXX);
--
-- Name: lead_XXXXXXXXXXXXXXXXXXXX; Type: INDEX; Schema: public; Owner: pod; Tablespace:
--
CREATE INDEX lead_XXXXXXXXXXXXXXXXXXXX ON lead USING btree (XXXXXXXXXXXXXXXXXXXX);
--
-- Name: lead_email; Type: INDEX; Schema: public; Owner: pod; Tablespace:
--
CREATE INDEX lead_email ON lead USING btree (email);
--
-- Name: lead_id_prefix; Type: INDEX; Schema: public; Owner: pod; Tablespace:
--
CREATE INDEX lead_id_prefix ON lead USING btree (id text_pattern_ops);
--
-- PostgreSQL database dump complete
--
いくつかの PG カタログの呪文:
db=> select * from pg_index where indexrelid = 'lead_email'::regclass;
indexrelid | indrelid | indnatts | indisunique | indisprimary | indisexclusion | indimmediate | indisclustered | indisvalid | indcheckxmin | indisready | indkey | indcollation | indclass | indoption | indexprs | indpred
------------+-----------+----------+-------------+--------------+----------------+--------------+----------------+------------+--------------+------------+--------+--------------+----------+-----------+----------+---------
215251995 | 101034456 | 1 | f | f | f | t | f | t | t | t | 101 | 100 | 10043 | 0 | ¤ | ¤
(1 row)
いくつかのロケール情報:
db=> show lc_collate;
lc_collate
-------------
en_US.UTF-8
(1 row)
db=> show lc_ctype;
lc_ctype
-------------
en_US.UTF-8
(1 row)
過去のSOの質問をかなり検索しましたが、このような単純な等式クエリに関するものはありませんでした。