Analyzing Queries Using Query Profile

Query Profile, available through the Snowflake web interface, provides execution details for a query. For the selected query, it provides a graphical representation of the main components of the processing plan for the query, with statistics for each component, along with details and statistics for the overall query.

In this Topic:

Introduction

When to Use Query Profile

Query Profile is a powerful tool for understanding the mechanics of queries. It can be used whenever you want or need to know more about the performance or behavior of a particular query. It is designed to help you spot typical mistakes in SQL query expressions to identify potential performance bottlenecks and improvement opportunities.

Additional high-level information about individual queries can be viewed in various columns in the Worksheet and History pages.

How to Access Query Profile

Query Profile is accessed from the detail page for a query. As such, you can access Query Profile from any page where the Query ID column is displayed and query IDs can be clicked on, specifically:

  • Worksheet
  • History

Note

If the Query ID column isn’t displayed on these pages, click the dropdown next to one of the column headers on the page and, in the list of Columns, select Query ID.

To access the profile for a query:

  1. In the History or Worksheet page, click on a query ID to display the detail page for the query:

    ../_images/ui-profile-detail.png
  2. Click the Profile tab. If the query has a profile, it is displayed (see screenshot below).

Query Profile Interface

For the purpose of this topic, we are using a simple SQL query that joins two tables:

select sum(v)
from fact join dim using (k)
where d < 100
and v > (
  select avg(v) from fact
);

The following screenshot shows the profile for this query:

../_images/ui-profile-step1.png

The interface consists of the following three main elements:

  • Steps (if the query was executed in multiple processing steps).
  • Operator tree on the left.
  • Query/operator detail panel on the right.

Steps

Some queries are executed in multiple processing steps. For example, our example query executes in 2 steps:

  • Step 1 computes the average of column fact.v.
  • Step 2 uses this intermediate result to compute the final query result.

Query Profile displays each processing step in a separate panel. You can switch between panels by clicking the respective step. For our example query, clicking Step 2 changes the view to:

../_images/ui-profile-step2.png

Operator Tree

The tree provides a graphical representation of the operators that comprise a query and the links that connect each operator:

  • Operators are the functional building blocks of a query. They are responsible for different aspects of data management and processing, including data access, transformations and updates. Each operator node in the tree includes some basic attributes:

    Type [#]:

    Operator type and ID number. ID can be used to uniquely identify an operator within a query profile. For example, TableScan [8] and JoinFilter [9] in the screenshot above.

    For descriptions of all the types, see Operator Types below.

    Percentage:

    Fraction of time that this operator consumed within the query step, e.g. 35.9% (for TableScan [8]). This information is also reflected in the orange bar at the bottom of the operator node, allowing for easy visual identification of performance-critical operators.

    Label:

    Operator-specific additional information, e.g. TESTDB.PUBLIC.FACT (for TableScan [8]).

  • Links represent the data flowing between each operator. Each link provides the number of records that were processed, e.g. 700M (from TableScan [8] to JoinFilter [9]).

Detail Panel

The detail panel on the right provides information about the selected components (operators and links) in the tree on the left:

  • Initially, no components in the tree are selected, so the panel shows information for the current step (see screenshot above).
  • When a component is selected by clicking on the node, the panel shows information for the component (see screenshot below).

Note

To return to the step-level information, simply click on any empty space around the operator tree.

The detail panel is divided into 3 sections:

Execution Time:Provides information about which processing tasks consumed query time (described in Query/Operator Details below). Additionally, for step-level information, it shows the state of the given step, and its execution time.
Statistics:Provides detailed information about various statistics (described in Query/Operator Details below).
Attributes:Provides component-specific information (described in Operator Types below).

The following screenshot shows the details after clicking the Join [11] operator:

../_images/ui-profile-operator.png

Operator Types

The following sections provide a list of the most common operator types and their attributes.

Data Access and Generation Operators

TableScan

Represents access to a single table. Attributes:

  • Full table name — the name of the accessed table, including database and schema.
  • Columns — list of scanned columns
  • Table alias — used table alias, if present
  • Extracted Variant paths — list of paths extracted from VARIANT columns
ValuesClause

List of values provided with the VALUES clause. Attributes:

  • Number of values — the number of produced values.
  • Values — the list of produced values.
Generator

Generates records using the TABLE(GENERATOR(...)) construct. Attributes:

  • rowCount — provided rowCount parameter.
  • timeLimit — provided timeLimit parameter.
ExternalScan

Represents access to data stored in stage objects. Can be a part of queries that scan data from stages directly, but also for data-loading COPY queries. Attributes:

  • Stage name — the name of the stage where the data is read from.
  • Stage type — the type of the stage, e.g. TABLE STAGE.
InternalObject

Represents access to an internal data object, e.g. an information schema table or the result of a previous query. Attributes:

  • Object Name — the name or type of the accessed object.

Data Processing Operators

Filter

Represents an operation that filters the records. Attributes:

  • Filter condition - the condition used to perform filtering.
Join

Combines two inputs on a given condition. Attributes:

  • Join Type — e.g. INNER, LEFT OUTER, etc.
  • Equality Join Condition — for joins which use equality-based conditions, it lists the expressions used for joining elements.
  • Additional Join Condition — some joins use conditions containing non-equality based predicates. They are listed here.

Note

Non-equality join predicates may result in significantly slower processing speeds and should be avoided if possible.

Aggregate

Groups input and computes aggregate functions. Can represent SQL constructs such as GROUP BY, as well as SELECT DISTINCT. Attributes:

  • Grouping Keys — if GROUP BY is used, this lists the expressions we group by.
  • Aggregate Functions — list of functions computed for each aggregate group, e.g. SUM.
GroupingSets

Represents constructs such as GROUPING SETS, ROLLUP and CUBE. Attributes:

  • Grouping Key Sets — list of grouping sets
  • Aggregate Functions — list of functions computed for each group, e.g. SUM.
WindowFunction

Computes window functions. Attributes:

  • Window Functions — list of window functions computed.
Sort

Orders input on a given expression. Attributes:

  • Sort keys — expression defining the sorting order.
SortWithLimit

Produces a part of the input sequence after sorting, typically a result of an ORDER BY .. LIMIT .. OFFSET .. construct in SQL. Attributes:

  • Sort keys — expression defining the sorting order.
  • Number of rows — number of rows produced.
  • Offset — position in the ordered sequence from which produced tuples are emitted.
Flatten

Processes VARIANT records, possibly flattening them on a specified path. Attributes:

  • input — the input expression used to flatten the data.
JoinFilter

Special filtering operation that removes tuples that can be identified as not possibly matching the condition of a Join further in the query plan. Attributes:

  • Original join ID — the join used to identify tuples that can be filtered out.
UnionAll
Concatenates two inputs. Attributes: none.

DML Operators

Insert

Adds records to a table either through an INSERT or COPY operation. Attributes:

  • Input expressions — which expressions are inserted.
  • Table names — names of tables that records are added to.
Delete

Removes records from a table. Attributes:

  • Table name — the name of the table that records are deleted from.
Update

Updates records in a table. Attributes:

  • Table name — the name of the updated table.
Merge

Performs a MERGE operation on a table. Attributes:

  • Full table name — the name of the updated table.
Unload

Represents a COPY operation that exports data from a table into a file in a stage. Attributes:

  • Location - the name of the stage where the data is saved.

Metadata Operators

Some queries include steps that are pure metadata/catalog operations rather than data-processing operations. These steps consist of a single operator. Some examples include:

DDL and Transaction Commands

Used for creating or modifying objects, session, transactions, etc. Typically, these queries are not processed by a virtual warehouse and result in a single-step profile that corresponds to the matching SQL statement. For example:

CREATE DATABASE | SCHEMA | ...

ALTER DATABASE | SCHEMA | TABLE | SESSION | ...

DROP DATABASE | SCHEMA | TABLE | ...

COMMIT

Table Creation Command

DDL command for creating a table, e.g.:

CREATE TABLE

Similar to other DDL commands, these queries result in a single-step profile; however, they can also be part of a multi-step profile, e.g. when used in a CTAS statement:

CREATE TABLE ... AS SELECT ...
Query Result Reuse
A query that reuses the result of a previous query.
Metadata-based Result

A query whose result is computed based purely on metadata, without accessing any data. These queries are not processed by a virtual warehouse. For example:

SELECT COUNT(*) FROM ...

SELECT CURRENT_DATABASE()

Miscellaneous Operators

Result

Returns the query result. Attributes:

  • List of expressions - the expressions produced.

Query/Operator Details

To help you analyze query performance, the detail panel provides two classes of profiling information:

  • Execution time, broken down into categories
  • Detailed statistics

In addition, attributes are provided for each operator (described in Operator Types in this topic).

Execution Time

Execution time provides information about “where the time was spent” during the processing of a query. Time spent can be broken down into the following categories, displayed in the following order:

  • Processing — time spent on data processing by the CPU.
  • Local Disk IO — time when the processing was blocked by local disk access.
  • Remote Disk IO — time when the processing was blocked by remote disk access.
  • Network Communication — time when the processing was waiting for the network data transfer.
  • Synchronization — various synchronization activities between participating processes.
  • Initialization — time spent setting up the query processing.

Statistics

A major source of information provided in the detail panel is the various statistics, grouped in the following sections:

  • IO — information about the input-output operations performed during the query:
    • Scan progress — the percentage of data scanned for a given table so far.
    • Bytes scanned — the number of bytes scanned so far.
    • Percentage scanned from cache — the percentage of data scanned from the local disk cache.
    • Bytes written — bytes written (e.g. when loading into a table).
    • Bytes written to result — bytes written to a result object.
    • Bytes read from result — bytes read from a result object.
    • External bytes scanned — bytes read from an external object, e.g. a stage.
  • DML — statistics for Data Manipulation Language (DML) queries:
    • Number of rows inserted — number of rows inserted into a table (or tables).
    • Number of rows updated — number of rows updated in a table.
    • Number of rows deleted — number of rows deleted from a table.
    • Number of rows unloaded — number of rows unloaded during data export.
    • Number of bytes deleted — number of bytes deleted from a table.
  • Pruning — information on the effects of table pruning:
    • Partitions scanned — number of partitions scanned so far.
    • Partitions total — total number of partitions in a given table.
  • Spilling — information about disk usage for operations where intermediate results do not fit in memory:
    • Bytes spilled to local storage — volume of data spilled to local disk.
    • Bytes spilled to remote storage — volume of data spilled to remote disk.
  • Network — network communication:
    • Bytes sent over the network — amount of data sent over the network.

Common Query Problems Identified by Query Profile

This section describes some of the problems you can identify and troubleshoot using Query Profile.

“Exploding” Joins

One of the common mistakes SQL users make is joining tables without providing a join condition (resulting in a “Cartesian Product”), or providing a condition where records from one table match multiple records from another table. For such queries, the Join operator produces significantly (often by orders of magnitude) more tuples than it consumes.

This can be observed by looking at the number of records produced by a Join operator, and typically is also reflected in Join operator consuming a lot of time.

UNION Without ALL

In SQL, it is possible to combine two sets of data with either UNION or UNION ALL constructs. The difference between them is that UNION ALL simply concatenates inputs, while UNION does the same, but also performs duplicate elimination.

A common mistake is to use UNION when only the UNION ALL semantics is needed. These queries show in Query Profile as a UnionAll operator with an extra Aggregate operator on top (which performs duplicate elimination).

Queries Too Large to Fit in Memory

For some operations (e.g. duplicate elimination for a huge dataset), the amount of memory available for the servers used to execute the operation might not be sufficient to hold intermediate results. As a result, the query processing engine will start spilling the data to local disk. If the local disk space is not sufficient, the spilled data is then saved to remote disks.

This spilling can have a profound effect on query performance (especially if remote disk is used for spilling). To alleviate this, we recommend:

  • Using a larger warehouse (effectively increasing the available memory/local disk space for the operation), and/or
  • Processing data in smaller batches.

Inefficient Pruning

Snowflake collects rich statistics on data allowing it not to read unnecessary parts of a table based on the query filters. However, for this to have an effect, the data storage order needs to be correlated with the query filter attributes.

The efficiency of pruning can be observed by comparing Partitions scanned and Partitions total statistics in the TableScan operators. If the former is a small fraction of the latter, pruning is efficient. If not, the pruning did not have an effect.

Of course, pruning can only help for queries that actually filter out a signification amount of data. If the pruning statistics do not show data reduction, but there is a Filter operator above TableScan which filters out a number of records, this might signal that a different data organization might be beneficial for this query.

For more information about pruning, see Understanding Snowflake Table Structures.