数据库类型转换导致SQL不走索引的案例

highgo=# EXPLAIN (ANALYZE, BUFFERS)
SELECT v.id as varchar_id, i.id as int_id, v.col1, i.col1
FROM table_varchar_id v
, table_int_id i
WHERE CAST(v.id AS BIGINT) = i.id
AND i.id BETWEEN 100000000000000000 AND 100000000005000000;
                                                                  QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------
 Hash Join  (cost=8.45..586.02 rows=7 width=53) (actual time=0.026..2.209 rows=5 loops=1)
   Hash Cond: ((v.id)::bigint = i.id)
   Buffers: shared hit=418
   ->  Seq Scan on table_varchar_id v  (cost=0.00..515.00 rows=10000 width=32) (actual time=0.004..0.806 rows=10000 loops=1) 
         Buffers: shared hit=415
   ->  Hash  (cost=8.38..8.38 rows=5 width=21) (actual time=0.012..0.012 rows=5 loops=1)
         Buckets: 1024  Batches: 1  Memory Usage: 9kB
         Buffers: shared hit=3
         ->  Index Scan using idx_table_int_id on table_int_id i  (cost=0.28..8.38 rows=5 width=21) (actual time=0.009..0.010 rows=5 loops=1)
               Index Cond: ((id >= '100000000000000000'::bigint) AND (id <= '100000000005000000'::bigint))
               Buffers: shared hit=3
 Planning Time: 0.117 ms
 Execution Time: 2.237 ms
(13 行记录)

->  Seq Scan on table_varchar_id v  (cost=0.00..515.00 rows=10000 width=32) (actual time=0.004..0.806 rows=10000 loops=1) 
      Buffers: shared hit=415

highgo=# alter table table_varchar_id alter COLUMN id type bigint using id::bigint;  --统一使用 bigint 类型

highgo=# analyze table_varchar_id;        --收集表的统计信息，保证被修改数据类型的 id 字段的索引立即生效

highgo=# create index idx_table_varchar_cast_id on table_varchar_id(cast(id as bigint));

highgo=# analyze table_varchar_id;        --收集表的统计信息，保证类型转换索引、表达式索引立即生效

highgo=# EXPLAIN (ANALYZE, BUFFERS)
SELECT v.id as varchar_id, i.id as int_id, v.col1, i.col1
FROM table_varchar_id v
, table_int_id i
WHERE v.id = i.id::varchar      --i.id 字段添加 varchar 类型转换，v.id 可以执行索引扫描
AND i.id BETWEEN 100000000000000000 AND 100000000005000000;
                                                                   QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------
 Nested Loop  (cost=0.57..49.97 rows=5 width=53) (actual time=0.021..0.029 rows=5 loops=1)
   Buffers: shared hit=16 read=2
   ->  Index Scan using idx_table_int_id on table_int_id i  (cost=0.28..8.38 rows=5 width=21) (actual time=0.003..0.004 rows=5 loops=1)
         Index Cond: ((id >= '100000000000000000'::bigint) AND (id <= '100000000005000000'::bigint))
         Buffers: shared hit=3
   ->  Index Scan using idx_table_varchar_id on table_varchar_id v  (cost=0.29..8.31 rows=1 width=32) (actual time=0.004..0.004 rows=1 loops=5)
         Index Cond: ((id)::text = ((i.id)::character varying)::text)
         Buffers: shared hit=13 read=2
 Planning Time: 0.123 ms
 Execution Time: 0.048 ms
(10 行记录)