数据行较大时不使用PostgreSQL索引

Question

Hi I'm curious about why index doesn't work when data rows are large even 100.

Here's select for 10 data:

mydb> explain select * from data where user_id=1;
+-----------------------------------------------------------------------------------+
| QUERY PLAN                                                                        |
|-----------------------------------------------------------------------------------|
| Index Scan using ix_data_user_id on data  (cost=0.14..8.15 rows=1 width=2043) |
| Index Cond: (user_id = 1)                                                         |
+-----------------------------------------------------------------------------------+
EXPLAIN

Here's select for 100 data:

mydb> explain select * from data where user_id=1;
+------------------------------------------------------------+
| QUERY PLAN                                                 |
|------------------------------------------------------------|
| Seq Scan on data  (cost=0.00..44.67 rows=1414 width=945) |
| Filter: (user_id = 1)                                      |
+------------------------------------------------------------+
EXPLAIN

How can index work when data rows are 100?

Answer 1

100 is not a large amount of data. Think 10,000 or 100,000 rows for a respectable amount.

To put it simply, records in a table are stored on data pages. A data page typically has about 8k bytes (it depends on the database and on settings). A major purpose of indexes is to reduce the number of data pages that need to be read.

If all the records in a table fit on one page, there is no need to reduce the number pages being read. The one page will be read. Hence, the index may not be particularly useful.