Explaining the Explain - Part 1 (Webcast)

7/31/2019 Explaining the Explain - Part 1 (Webcast)

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Explaining the EXPLAIN Part 1

Joe Ramon

October 21, 2008


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Agenda

What is the EXPLAIN facility?

Where does the EXPLAIN output come FROM?

What does the Optimizer need to build a plan?

What does the EXPLAIN terminology mean?

What can be learned by reading the EXPLAIN text?

What can be done to influence the Optimizer?

Summary


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What is EXPLAIN?

The EXPLAIN facility provides an "English" translation of theplan the SQL Optimizer develops to service a request.

May be used on any SQL statement, except EXPLAIN itself.

Look for key words AND phrases

Execution time AND row count estimates depend on:> Are statistics collected?

Actual execution time depends on:

> Is DBS processing other requests?

> Is channel or network busy?


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How is EXPLAIN Text Generated?

STATISTICS

SQL REQUEST

DD

Dbase

AccessRights

RoleGrants (V2R5)

TVM

TVFields

Indexes

SYNTAXER

RESOLVER

SECURITY

OPTIMIZER

GENERATOR

DISPATCHER

AMP

DD Cache

EXPLAIN

APPLY


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Information Known to Optimizer

Number of nodes in system Number and type of CPUs per node

Number of configured AMP Vprocs

Disk array configuration

Interconnect configuration

Amount and configuration of memory

All are taken into account when calculating query cost.


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Columns with indexes Rows in the table

Rows per block

Values per column

Rows per value

Row length

Additional Information Required by the Optimizer


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Optimizer Random AMP Samples

Statistics collected by a random AMP sample apply in these cases row counts for the table are needed and statistics are not collected

on PI.

indexed columns are used in the query and statistics are notcollected.

With Teradata 12.0, statistics have been collected, but areconsidered stale.

By default, Teradata chooses an AMP for random AMP (or dynamic) datasampling.

Enhancement starting with Teradata 6.0.

When statistics are not available, the Optimizer can obtain randomsamples from more than one AMP when generating row counts for a

query plan.

Random AMP sampling is controlled via a DBS Control parameter.


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Random AMP Sampling

For a table row count estimate, read one cylinder on 1 AMP. Calculate the approximate number of rows in the table:

For NUSI estimates, read one cylinder from the NUSI subtable.

Uses a similar technique by counting the number of NUSI values inthe cylinder. The table row count is divided by the extrapolated

NUSI row count to get a rows/NUSI value.

Any skewed component in the sample skews the demographics.

For non-indexed columns without statistics, the optimizer uses fixedformulas to estimate the number of rows. For example,

Assumes 10% for one column in an equality condition

Assumes 7.5% for two columns, each in an equality condition, andANDed together


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Cost-based Optimizer - looks for lowest cost plan

Does not store plan - dynamically regenerates

As data demographics change, so may plan

Will only assign cost to steps for which there are choices

Assigns confidence factors on row estimates

Mature, large-table, decision-support optimization

Optimizer Facts


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EXPLAIN Example

EXPLAINSELECT Last_Name, First_Name, Dept_Name, Job_Desc

FROM Employee E

INNER JOIN Department D ON E.Dept_Number = D.Dept_Number

INNER JOIN Job J ON E.Job_code = J.Job_codeORDER BY 3, 1, 2;


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EXPLAIN Example (cont.)

1) First, we lock a distinct TFACT."pseudo table" for read on a RowHash to prevent globaldeadlock for TFACT.E.

2) Next, we lock a distinct TFACT."pseudo table" for read on a RowHash to prevent global

deadlock for TFACT.J.

3) We lock a distinct TFACT."pseudo table" for read on a RowHash to prevent global deadlock for

TFACT.D.

4) We lock TFACT.E for read, we lock TFACT.J for read, and we lock TFACT.D for read.

5) We execute the following steps in parallel.

1) We do an all-AMPs RETRIEVE step from TFACT.D by way of an all-rows scan with noresidual conditions into Spool 2 (all_amps), which is duplicated on all AMPs. The size of

Spool 2 is estimated with high confidence to be 19,642 rows (726,754 bytes). The

estimated time for this step is 0.02 seconds.

2) We do an all-AMPs RETRIEVE step from TFACT.J by way of an all-rows scan with no

residual conditions into Spool 3 (all_amps), which is duplicated on all AMPs. Then we do

a SORT to order Spool 3 by the hash code of (TFACT.J.Job_Code). The size of Spool 3 is

estimated with high confidence to be 12,166 rows (450,142 bytes). The estimated time for

this step is 0.01 seconds.

6) We do an all-AMPs JOIN step from Spool 2 (Last Use) by way of an all-rows scan, which is

joined to TFACT.E by way of an all-rows scan with a condition of ("NOT (TFACT.E.Job_Code IS

NULL)"). Spool 2 and TFACT.E are joined using a single partition hash_ join, with a join

: :

8) Finally, we send out an END TRANSACTION step to all AMPs involved in processing the

request.

-> The contents of Spool 1 are sent back to the user as the result of statement 1. The totalestimated time is 0.14 seconds.


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.Pseudo Table Locks.

Prevents two users from getting conflicting locks with all-

AMP requests

All-AMP lock requests are handled as follows:

PE determines Table ID hash for an AMP to manage the

all-AMP lock request.

Put pseudo lock on the tableAcquire lock on all AMPs

EXPLAIN Terminology


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AMP

PE PE

AMP AMP AMP

Determine

Table ID hash

First

request

Second

request

. Pseudo Table Locks.

EXPLAIN Terminology (cont.)


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Most EXPLAIN text is easy to understand. The following additional definitionsmay help:

... (Last Use)

A spool file is no longer needed and will be released when this step completes.

... with no residual conditions

All applicable conditions have been applied to the rows.

... END TRANSACTION

Transaction locks are released, and changes are committed.

... by way of the sort key in spool field1 (dbname.tablename.colname)

Field1 is created to allow a tag sort. Teradata 12.0 includes the column name used for

the sort.


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5) We execute the following steps in parallel.

1) We do an all-AMPs RETRIEVE step from TFACT.D by way of an all-rows scan with noresidual conditions into Spool 2 (all_amps), which is duplicated on all AMPs. The size ofSpool 2 is estimated with high confidence to be 19,642 rows (726,754 bytes). Theestimated time for this step is 0.02 seconds.

2) We do an all-AMPs RETRIEVE step from TFACT.J by way of an all-rows scan with noresidual conditions into Spool 3 (all_amps), which is duplicated on all AMPs. Then we doa SORT to order Spool 3 by the hash code of (TFACT.J.Job_Code). The size of Spool 3 isestimated with high confidence to be 12,166 rows (450,142 bytes). The estimated time for

this step is 0.01 seconds.6) We do an all-AMPs JOIN step from Spool 2 (Last Use) by way of an all-rows scan, which is


NULL)"). Spool 2 and TFACT.E are joined using a single partition hash_ join, with a join with a

join condition of ("TFACT.E.Dept_Number = Dept_Number"). The result goes into Spool 4

(all_amps), which is built locally on the AMPs. Then we do a SORT to order Spool 4 by the

hash code of (TFACT.E.Job_Code). The size of Spool 4 is estimated with low confidence to be

26,000 rows (1,690,000 bytes). The estimated time for this step is 0.04 seconds.7) We do an all-AMPs JOIN step from Spool 3 (Last Use) by way of a RowHash match scan, which

is joined to Spool 4 (Last Use) by way of a RowHash match scan. Spool 3 and Spool 4 are

joined using a merge join, with a join condition of ("Job_Code = Job_Code"). The result goes

into Spool 1 (group_amps), which is built locally on the AMPs. Then we do a SORT to order

Spool 1 by the sort key in spool field1 (TFACT.D.Dept_Name, TFACT.E.Last_Name,

TFACT.E.First_Name). The size of Spool 1 is estimated with low confidence to be 26,000 rows

(3,822,000 bytes). The estimated time for this step is 0.08 seconds.

8) Finally, we send out an END TRANSACTION step to all AMPs involved in processing therequest.



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Most EXPLAIN text is easy to understand. The following additional definitions mayhelp:

... we do an ABORT test

Caused by an ABORT or ROLLBACK statement.

... We execute these steps in parallel.

The following indented steps are executed in parallel.

3) We lock DBC.ArchiveLoggingObjsTbl for read on a RowHash, we lock DBC.TVM for write on

a RowHash, we lock DBC.TVFields for write on a RowHash, we lock DBC.Indexes for write on

a RowHash, we lock DBC.DBase for read on a RowHash, and we lock DBC.AccessRights for

write on a RowHash.4) We execute the following steps in parallel.

1) We do a single-AMP ABORT test from DBC.ArchiveLoggingObjsTbl by way of the

primary index.2) We do a single-AMP ABORT test from DBC.DBase by way of the unique primary index.

3) We do a single-AMP ABORT test from DBC.TVM by way of the unique primary index.

4) We do an INSERT into DBC.TVFields (no lock required).

: :

7) We do an INSERT into DBC.Indexes (no lock required).

8) We do an INSERT into DBC.TVM (no lock required).

9) We INSERT default rights to DBC.AccessRights for TFACT.Orders.



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... which is redistributed by hash code to all AMPs (dbname.tablename.colname)

Redistributing data (in SPOOL) in preparation for a join. Teradata 12.0 includes the column

name.

... which is duplicated on all AMPs

Duplicating data (in SPOOL) from the smaller table in preparation for a join.

... (one_amp) or (group_amps) or (all_amps) Indicates one AMP, a subset of AMPs, or all of the AMPs will participate.

... ("NOT (table_name.column_name IS NULL)")

Feature where optimizer realizes that the column being joined to is NOT NULL or has

referential integrity.

... eliminating duplicate rows ...

Duplicate rows only exist in spool files, not set tables.



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4) We do an all-AMPsRETRIEVE step from TFACT.D by way of an all-rows scan with no residualconditions into Spool 2 (all_amps), which is duplicated on all AMPs. The size of Spool 2 isestimated with high confidence to be 19,642 rows (726,754 bytes). The estimated time for thisstep is 0.02 seconds.

5) We do an all-AMPs JOIN step from Spool 2 (Last Use) by way of an all-rows scan, which isjoined to TFACT.E by way of an all-rows scan. Spool 2 and TFACT.E are joined using a singlepartition hash_ join, with a join condition of ("TFACT.E.Dept_Number = Dept_Number"). Theresult goes into Spool 1 (group_amps), which is redistributed by the hash code of

(TFACT.E.First_Name, TFACT.E.Last_Name, TFACT.E.Employee_Number,TFACT.D.Dept_Name) to all AMPs. The size of Spool 1 is estimated with low confidence to be26,000 rows (3,614,000 bytes). The estimated time for this step is 0.09 seconds.

6) We do an all-AMPs RETRIEVE step from TFACT.D by way of an all-rows scan with a condition of("NOT (TFACT.D.Dept_Mgr_Number IS NULL)") into Spool 3 (all_amps), which is redistributedby the hash code of (TFACT.D.Dept_Mgr_Number) to all AMPs. Then we do a SORT to orderSpool 3 by row hash. The size of Spool 3 is estimated with high confidence to be 1,403 rows(51,911 bytes). The estimated time for this step is 0.01 seconds.

7) We do an all-AMPs JOIN step from Spool 3 (Last Use) by way of a RowHash match scan, whichis joined to TFACT.E by way of a RowHash match scan with no residual conditions. Spool 3and TFACT.E are joined using a merge join, with a join condition of("TFACT.E.Employee_Number = Dept_Mgr_Number"). The result goes into Spool 1(group_amps), which is redistributed by the hash code of (TFACT.E.First_Name,TFACT.E.Last_Name, TFACT.E.Employee_Number, TFACT.D.Dept_Name) to all AMPs. Then wedo a SORT to order Spool 1 by the sort key in spool field1 eliminating duplicate rows. The sizeof Spool 1 is estimated with low confidence to be 27,403 rows (3,809,017 bytes). The estimated

time for this step is 0.06 seconds.



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... we do a BMSMS (bit map set manipulation step) Doing a NUSI Bit Map operation.

:

3) We do a BMSMS (bit map set manipulation) step that builds a bit map for TFACT.Employee

by way of index # 4 "TFACT.E.Job_Code = 3500" which is placed in Spool 2. The estimated

time for this step is 0.01 seconds.

4) We do an all-AMPs RETRIEVE step from TFACT.E by way of index # 8 TFACT.E.Dept_Number= 1310" and the bit map in Spool 2 (Last Use) with a residual condition of

("TFACT.E.Job_Code = 3500") into Spool 1 (group_amps), which is built locally on the AMPs.

The size of Spool 1 is estimated with low confidence to be 60 rows (4620 bytes). The


5) Finally, we send out an END TRANSACTION step to all AMPs involved in processing the

request.

-> The contents of Spool 1 are sent back to the user as the result of statement 1. The totalestimated time is 0.03 seconds.

Note:

Statistics were collected on the NUSIs Job_Code and Dept_Number.



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Synchronized Scanning

In the case of multiple users that access the same table at the same time,the system can do a synchronized scan (sync scan) on the table.

112747 1766 100766 3001 Frankel Allan

034982 2212 106363 3005 Bench John

310229 2231 108222 3100 Palmer Carson

209181 1235 108221 3001 Smith Buster

123881 2433 101433 3007 Walton Sam

223431 2500 105200 3101 Brooks Steve

221015 1019 108222 3199 Woods Tiger

121332 2281 101281 3007 Walton John

118314 2100 101100 3002 Ramon Anne

104631 1279 100279 3002 Roberts Julie

210110 1201 101222 3003 Douglas Michael210001 1205 105432 3022 Morgan Joe

100076 1011 104321 3021 Anderson Sparky

100045 1012 101231 3087 Michelson Phil

319116 1219 121871 3025 Crawford Cindy

: : : : : :

: : : : : :

Query 1

Begins

Query 2

Begins

Query 3

Begins


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EXPLAIN SELECT * FROM daily_sales ORDER BY 1;

Synchronized Scanning (cont.)

:

3) We do an all-AMPs RETRIEVE step from TFACT.daily_sales by way of an all-rows scan with

no residual conditions into Spool 1 (group_amps), which is built locally on the AMPs.

Then we do a SORT to order Spool 1 by the sort key in spool field1

(TFACT.daily_sales.Item_id). The input table will not be cached in memory, but it is eligible

for synchronized scanning. The result spool file will not be cached in memory. The size ofSpool 1 is estimated with high confidence to be 76,685 rows (2,530,605 bytes). The


:


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The EXPLAIN facility may express confidence for a retrievefrom a table.

Some of the phrases used are:

. .. with high confidence . . .

Restricting conditions exist on index(es) or column(s) that have collectedstatistics.

. . . with low confidence . . .

Restricting conditions exist on index(es) having no statistics, but estimates

can be based upon a sampling of the index(es).

Restricting conditions exist on index(es) or column(s) that have collected

statistics but are AND-ed together with conditions on non-indexedcolumns.

Restricting conditions exist on index(es) or column(s) that have collected

statistics but are OR-ed together with other conditions.

. . . with no confidence . . .

Conditions outside the above.

Understanding Row and Time Estimates


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The following are confidence phrases for a join:

. . . with index join confidence . . . A join condition via a primary index.

. . . with high confidence . . . One input relation has high confidence and the other has high or index join

confidence.

. . . with low confidence . . . One input relation has low confidence and the other has low, high, or join

index confidence.

. . . with no confidence . . . One input relation has no confidence. Statistics do not exist for either join field.



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Understanding Row and Time Estimates (cont.)

5) We execute the following steps in parallel.1) We do an all-AMPs RETRIEVE step from TFACT.D by way of an all-rows scan with no

residual conditions into Spool 2 (all_amps), which is duplicated on all AMPs. The size ofSpool 2 is estimated with high confidence to be 19,642 rows (726,754 bytes). Theestimated time for this step is 0.02 seconds.

2) We do an all-AMPs RETRIEVE step from TFACT.J by way of an all-rows scan with noresidual conditions into Spool 3 (all_amps), which is duplicated on all AMPs. Then we doa SORT to order Spool 3 by the hash code of (TFACT.J.Job_Code). The size of Spool 3 isestimated with high confidence to be 12,166 rows (450,142 bytes). The estimated time forthis step is 0.01 seconds.

6) We do an all-AMPs JOIN step from Spool 2 (Last Use) by way of an all-rows scan, which is


NULL)"). Spool 2 and TFACT.E are joined using a single partition hash_ join, with a join with a

join condition of ("TFACT.E.Dept_Number = Dept_Number"). The result goes into Spool 4

(all_amps), which is built locally on the AMPs. Then we do a SORT to order Spool 4 by the

hash code of (TFACT.E.Job_Code). The size of Spool 4 is estimated with low confidence to be

26,000 rows (1,690,000 bytes). The estimated time for this step is 0.04 seconds.7) We do an all-AMPs JOIN step from Spool 3 (Last Use) by way of a RowHash match scan, which

is joined to Spool 4 (Last Use) by way of a RowHash match scan. Spool 3 and Spool 4 are

joined using a merge join, with a join condition of ("Job_Code = Job_Code"). The result goes

into Spool 1 (group_amps), which is built locally on the AMPs. Then we do a SORT to order

Spool 1 by the sort key in spool field1 (TFACT.D.Dept_Name, TFACT.E.Last_Name,

TFACT.E.First_Name). The size of Spool 1 is estimated with low confidence to be 26,000 rows

(3,822,000 bytes). The estimated time for this step is 0.08 seconds.8) Finally, we send out an END TRANSACTION step to all AMPs involved in processing the

request.


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Row estimates:> May be estimated using random samples, statistics or indexes> Are assigned a confidence level - high, low or none> Affect timing estimates - more rows, more time needed

Timings:

> Used to determine the lowest cost plan> Total cost generated if all processing steps have assigned cost> Not intended to predict wall-clock time, useful for comparisons

Miscellaneous Notes:

>Estimates too large to display show 3 asterisks (***).

> The accuracy of the time estimate depends upon the accuracy ofthe row estimate.

Query Cost Estimates


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Low and no confidence may indicate a need to collectstatistics on indexes or columns involved in restrictingconditions.

You may otherwise consider a closer examination of theconditions in the query for possible changes that may

improve the confidence.

Collecting statistics or altering the conditions has no realimpact unless it influences the optimizer to pick a better plan.



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EXPLAIN of Create Table

EXPLAIN CREATE TABLE Orders(order_id INTEGER NOT NULL

,order_date DATE FORMAT 'yyyy-mm-dd'

,cust_id INTEGER)

UNIQUE PRIMARY INDEX (order_id);

1) First, we lock TFACT.Orders for exclusive use.

:4) We execute the following steps in parallel.1) We do a single-AMP ABORT test from DBC.ArchiveLoggingObjsTbl by way of the

primary index.2) We do a single-AMP ABORT test from DBC.DBase by way of the unique primary index.3) We do a single-AMP ABORT test from DBC.TVM by way of the unique primary index.4) We do an INSERT into DBC.TVFields (no lock required).5) We do an INSERT into DBC.TVFields (no lock required).

6) We do an INSERT into DBC.TVFields (no lock required).7) We do an INSERT into DBC.Indexes (no lock required).8) We do an INSERT into DBC.TVM (no lock required).9) We INSERT default rights to DBC.AccessRights for TFACT.Orders.

5) We create the table header.6) Finally, we send out an END TRANSACTION step to all AMPs involved in processing the

request.-> No rows are returned to the user as the result of statement 1.


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Unique Primary INDEX Request (UPI)

EXPLAINSELECT *

FROM Employee

WHERE Employee_Number = 1104066;

Simplest and most efficient type of access.

Spool is not used.

1) First, we do a single-AMP RETRIEVE step from TFACT.Employee byway of the unique primary index "TFACT.Employee.Employee_Number

= 1104066" with no residual conditions. The estimated time for this step

is 0.00 seconds.

-> The row is sent directly back to the user as the result of statement 1.

The total estimated time is 0.00 seconds.


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Residual condition does not help the query.

No change to plan or time estimate.

UPI Request With Residual Condition

EXPLAINSELECT *

FROM Employee

WHERE Employee_Number = 1104066

AND Dept_Number = 1404;

1) First, we do a single-AMP RETRIEVE step from TFACT.Employee by

way of the unique primary index "TFACT.Employee.Employee_Number =

1104066" with a residual condition of ("TFACT.Employee.Dept_Number =

1404"). The estimated time for this step is 0.00 seconds.

-> The row is sent directly back to the user as the result of statement 1. The

total estimated time is 0.00 seconds.


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Full Table Scan

EXPLAINSELECT *

FROM Employee

WHERE Emp_Mgr_Number = 104043

AND Job_Code = 3405;

1) First, we lock a distinct TFACT."pseudo table" for read on a RowHash toprevent global deadlock for TFACT.Employee.

2) Next, we lock TFACT.Employee for read.

3) We do an all-AMPs RETRIEVE step from TFACT.Employee by way of an all-

rows scan with a condition of ("(TFACT.Employee.Emp_Mgr_Number =

104043) AND (TFACT.Employee.Job_Code = 3405)") into Spool 1

(group_amps), which is built locally on the AMPs. The size of Spool 1 is

estimated with low confidence to be 8 rows (616 bytes). The estimated timefor this step is 0.02 seconds.

4) Finally, we send out an END TRANSACTION step to all AMPs involved in

processing the request.

-> The contents of Spool 1 are sent back to the user as the result of statement

1.



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Aggregations

EXPLAIN SELECT dept_number, SUM(salary_amount)FROM Employee GROUP BY 1;

1) First, we lock a distinct TFACT."pseudo table" for read on a RowHash to

prevent global deadlock for TFACT.employee.

2) Next, we lock TFACT.employee for read.

3) We do an all-AMPs SUM step to aggregate from TFACT.employee by way of anall-rows scan with no residual conditions, grouping by field1

(TFACT.employee.Dept_Number). Aggregate Intermediate Results are

computed globally, then placed in Spool 3. The size of Spool 3 is estimated

with high confidence to be 1,403 rows (51,911 bytes). The estimated time for


4) We do an all-AMPs RETRIEVE step from Spool 3 (Last Use) by way of an all-

rows scan into Spool 1 (group_amps), which is built locally on the AMPs. The

size of Spool 1 is estimated with high confidence to be 1,403 rows (57,523

bytes). The estimated time for this step is 0.02 seconds.



-> The contents of Spool 1 are sent back to the user as the result of statement 1.



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INSERT/SELECT is the process of SELECTing data FROM onetable and using it as input to be inserted into another table.

Two different optimizations can occur:

1) If the PI of the source AND destination tables are identical, an

AMP local operation is used.2) If the target table is empty,

a) Transient Journaling is reduced

b) 127 KB block transfers are used

If both conditions are satisfied, both optimizations are used.

Optimized INSERT/SELECT


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Optimized INSERT/SELECT Example

1) First, we lock a distinct TFACT."pseudo table" for write on a RowHash to

prevent global deadlock for TFACT.Employee_copy.

2) Next, we lock a distinct TFACT."pseudo table" for read on a RowHash toprevent global deadlock for TFACT.Employee.

3) We lock TFACT.Employee_copy for write, and we lock TFACT.Employee for

read.

4) We do an all-AMPs MERGE intoTFACT.Employee_copy from

TFACT.Employee. The size is estimated with no confidence to be 25,382 rows.

The estimated time for this step is 2.29 seconds.

5) We spoil the parser's dictionary cache for the table.6) Finally, we send out an END TRANSACTION step to all AMPs involved in


-> No rows are returned to the user as the result of statement 1.

EXPLAININSERTINTO Employee_copy SELECT * FROM Employee;


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If the target table has a different Primary Index, astandard insert SELECT process must be used.

A BYNET operation will be used to relocate the SELECTedrows onto the target AMPs.

This will require:

a) Single row inserts (vs. 127 KB blocks)

b) Transient journal entries for each row

INSERT/SELECT With Different PIs


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Non-Optimized INSERT/SELECT Example

CREATESETTABLE TFACT.Employee_CharPI

(Employee_Number CHAR(10),

Location_Number INTEGER,:

Salary_Amount DECIMAL(10,2))

UNIQUEPRIMARYINDEX ( Employee_Number );

CREATESETTABLE TFACT.Employee

(Employee_Number INTEGER,

Location_Number INTEGER,

:

Salary_Amount DECIMAL(10,2))

UNIQUEPRIMARYINDEX ( Employee_Number );


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Non-Optimized INSERT/SELECT Example (cont.)

EXPLAININSERTINTO Employee_CharPI SELECT * FROM Employee;

:

4) We do an all-AMPs RETRIEVE step from TFACT.Employee by way of an all-rows

scan with no residual conditions into Spool 1 (all_amps), which is redistributed

by the hash code of (TFACT.Employee.Employee_Number (CHAR(10),

CHARACTER SET LATIN, NOT CASESPECIFIC, FORMAT '-(10)9')(CHAR(10),

CHARACTER SET LATIN, NOT CASESPECIFIC, NAMED Employee_Number,

FORMAT 'X(10)', NULL)) to all AMPs. Then we do a SORT to order Spool 1 by

row hash. The size of Spool 1 is estimated with high confidence to be 26,000

rows (1,950,000 bytes). The estimated time for this step is 0.06 seconds.

5) We do an all-AMPs MERGE into TFACT.Employee_CharPI from Spool 1 (Last

Use). The size is estimated with high confidence to be 26,000 rows. Theestimated time for this step is 1.38 seconds.

6) We spoil the parser's dictionary cache for the table.



-> No rows are returned to the user as the result of statement 1.


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Unexpected Full Table Scan

EXPLAINSELECT * FROM Employee_CharPI WHERE employee_number = 1104066 ;

1) First, we lock a distinct TFACT."pseudo table" for read on a RowHash to prevent

global deadlock for TFACT.Employee_CharPI.

2) Next, we lock TFACT.Employee_CharPI for read.

3) We do an all-AMPs RETRIEVE step from TFACT.Employee_CharPI by way of anall-rows scanwith a condition of ("(TFACT.Employee_CharPI.Employee_Number

(FLOAT, FORMAT'-9.99999999999999E-999'))= 1.10406600000000E 006") into

Spool 1 (group_amps), which is built locally on the AMPs. The size of Spool 1 is

estimated with no confidence to be 2 rows (166 bytes). The estimated time for



processing the request.-> The contents of Spool 1 are sent back to the user as the result of statement 1.



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Correct use of Primary INDEX

EXPLAINSELECT * FROM Employee_CharPI WHERE employee_number = '1104066' ;

1) First, we do a single-AMP RETRIEVE step from

TFACT.Employee_CharPI by way of the unique primary index

"TFACT.Employee_CharPI.Employee_Number = '1104066 '" with noresidual conditions. The estimated time for this step is 0.00 seconds.

-> The row is sent directly back to the user as the result of statement 1.



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EXPLAIN EXEC Dept_List (1404);

Explaining Macros

CREATEMACRO Dept_List (dept_no INTEGER)AS

(SELECT * FROM Employee WHERE dept_number = :dept_no;);

1) First, we lock a distinct TFACT."pseudo table" for read on a RowHash toprevent global deadlock for TFACT.Employee.



rows scan with a condition of ("TFACT.Employee.Dept_Number = 1404") into

Spool 1 (group_amps), which is built locally on the AMPs. The size of Spool

1 is estimated with high confidence to be 40 rows (3,080 bytes). The




-> The contents of Spool 1 are sent back to the user as the result of statement 1.



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Explaining Macros (cont.)

EXPLAINUSING (dept_no INTEGER) EXECUTE Dept_List (:dept_no);

1) First, we lock a distinct TFACT."pseudo table" for read on a RowHash to

prevent global deadlock for TFACT.Employee.



rows scan with a condition of ("TFACT.Employee.Dept_Number =

:dept_no") into Spool 1 (group_amps), which is built locally on the AMPs.

The size of Spool 1 is estimated with low confidence to be 19 rows (1,463

bytes). The estimated time for this step is 0.02 seconds.



-> The contents of Spool 1 are sent back to the user as the result of statement 1.The total estimated time is 0.02 seconds.


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COLLECTED STATISTICS can help the Optimizer make betterdecisions using actual row counts and data distributioninformation.

Collect Statistics on:

Non-unique indexes

Non-index join columns

Primary Index of small tables

Collect Statistics considerations: Requires a full table scan

Must be kept current

May be unnecessary for very

large tables

How to Influence the Optimizer


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Proper index choices at physical design time Add secondary, join, or hash indexes where helpful

Use equality-based join conditions

Experiment using EXPLAIN

Other Factors To Help Optimizer


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EXPLAIN is a tool to help you plan query resources Teradata uses a cost-based optimizer

Adding Secondary, Join, or Hash Indexes gives optimizermore choices

Collecting Statistics allows better plan estimates

Most mature optimizer for mixed workload environments inthe industry

Summary


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Documents

Explaining the Explain - Part 1 (Webcast)