Amazon Redshift will no longer support the creation of new Python UDFs starting Patch 198. Existing Python UDFs will continue to function until June 30, 2026. For more information, see the [ blog post ](https://aws.amazon.com/blogs/big-data/amazon-redshift-python-user-defined-functions-will-reach-end-of-support-after-june-30-2026/). 

# STL\$1SCAN
<a name="r_STL_SCAN"></a>

Analyzes table scan steps for queries. The step number for rows in this table is always 0 because a scan is the first step in a segment.

STL\$1SCAN is visible to all users. Superusers can see all rows; regular users can see only their own data. For more information, see [Visibility of data in system tables and views](cm_chap_system-tables.md#c_visibility-of-data).

**Note**  
STL\$1SCAN only contains queries run on main provisioned clusters. It doesn't contain queries run on concurrency scaling clusters or on serverless namespaces. To access explain plans for queries run on both main clusters, concurrency scaling clusters, and serverless namespaces, we recommend that you use the SYS monitoring view [SYS\$1QUERY\$1DETAIL](SYS_QUERY_DETAIL.md) . The data in the SYS monitoring view is formatted to be easier to use and understand.

## Table columns
<a name="r_STL_SCAN-table-columns"></a>

[\[See the AWS documentation website for more details\]](http://docs.aws.amazon.com/redshift/latest/dg/r_STL_SCAN.html)

## Scan types
<a name="r_STL_SCAN-scan-types"></a>

[\[See the AWS documentation website for more details\]](http://docs.aws.amazon.com/redshift/latest/dg/r_STL_SCAN.html)

## Usage notes
<a name="w2aac59c29b9d101c15"></a>

Ideally `rows` should be relatively close to `rows_pre_filter`. A large difference between `rows` and `rows_pre_filter` is an indication that the execution engine is scanning rows that are later discarded, which is inefficient. The difference between `rows_pre_filter` and `rows_pre_user_filter` is the number of ghost rows in the scan. Run a VACUUM to remove rows marked for deletion. The difference between `rows` and `rows_pre_user_filter` is the number of rows filtered by the query. If a lot of rows are discarded by the user filter, review your choice of sort column or, if this is due to a large unsorted region, run a vacuum.

## Sample queries
<a name="r_STL_SCAN-sample-queries"></a>

The following example shows that `rows_pre_filter` is larger than `rows_pre_user_filter` because the table has deleted rows that have not been vacuumed (ghost rows). 

```
SELECT query, slice, segment,step,rows, rows_pre_filter, rows_pre_user_filter 
from stl_scan where query = pg_last_query_id();

 query |  slice | segment | step | rows  | rows_pre_filter | rows_pre_user_filter
-------+--------+---------+------+-------+-----------------+----------------------
 42915 |      0 |       0 |    0 | 43159 |           86318 |                43159
 42915 |      0 |       1 |    0 |     1 |               0 |                    0
 42915 |      1 |       0 |    0 | 43091 |           86182 |                43091
 42915 |      1 |       1 |    0 |     1 |               0 |                    0
 42915 |      2 |       0 |    0 | 42778 |           85556 |                42778
 42915 |      2 |       1 |    0 |     1 |               0 |                    0
 42915 |      3 |       0 |    0 | 43428 |           86856 |                43428
 42915 |      3 |       1 |    0 |     1 |               0 |                    0
 42915 |  10000 |       2 |    0 |     4 |               0 |                    0
(9 rows)
```