Optimizing Statistics

What Is Statistic Optimization

GaussDB(DWS) generates optimal execution plans based on the cost estimation. Optimizers need to estimate the number of data rows and the cost based on statistics collected using ANALYZE. Therefore, the statistics is vital for the estimation of the number of rows and cost. Global statistics are collected using ANALYZE: relpages and reltuples in the pg_class table; stadistinct, stanullfrac, stanumbersN, stavaluesN, and histogram_bounds in the pg_statistic table.

Example 1: Poor Query Performance Due to the Lack of Statistics

The query performance is often significantly impacted by the absence of statistics for tables or columns involved in the query.

The structure of the example table is as follows:

    
     
       
       
         CREATE TABLE LINEITEM
(
L_ORDERKEY         BIGINT        NOT NULL
, L_PARTKEY        BIGINT        NOT NULL
, L_SUPPKEY        BIGINT        NOT NULL
, L_LINENUMBER     BIGINT        NOT NULL
, L_QUANTITY       DECIMAL(15,2) NOT NULL
, L_EXTENDEDPRICE  DECIMAL(15,2) NOT NULL
, L_DISCOUNT       DECIMAL(15,2) NOT NULL
, L_TAX            DECIMAL(15,2) NOT NULL
, L_RETURNFLAG     CHAR(1)       NOT NULL
, L_LINESTATUS     CHAR(1)       NOT NULL
, L_SHIPDATE       DATE          NOT NULL
, L_COMMITDATE     DATE          NOT NULL
, L_RECEIPTDATE    DATE          NOT NULL
, L_SHIPINSTRUCT   CHAR(25)      NOT NULL
, L_SHIPMODE       CHAR(10)      NOT NULL
, L_COMMENT        VARCHAR(44)   NOT NULL
) with (orientation = column, COMPRESSION = MIDDLE) distribute by hash(L_ORDERKEY);

CREATE TABLE ORDERS
(
O_ORDERKEY        BIGINT        NOT NULL
, O_CUSTKEY       BIGINT        NOT NULL
, O_ORDERSTATUS   CHAR(1)       NOT NULL
, O_TOTALPRICE    DECIMAL(15,2) NOT NULL
, O_ORDERDATE     DATE NOT NULL
, O_ORDERPRIORITY CHAR(15)      NOT NULL
, O_CLERK         CHAR(15)      NOT NULL
, O_SHIPPRIORITY  BIGINT        NOT NULL
, O_COMMENT       VARCHAR(79)   NOT NULL
)with (orientation = column, COMPRESSION = MIDDLE) distribute by hash(O_ORDERKEY);

        

      

    
   

The query statements are as follows:

    
     
       
       
         explain verbose select
count(*) as numwait 
from
lineitem l1,
orders 
where
o_orderkey = l1.l_orderkey
and o_orderstatus = 'F'
and l1.l_receiptdate > l1.l_commitdate
and not exists (
select
*
from
lineitem l3
where
l3.l_orderkey = l1.l_orderkey
and l3.l_suppkey <> l1.l_suppkey
and l3.l_receiptdate > l3.l_commitdate
)
order by
numwait desc;

        

      

    
   

You can perform the following operations to check whether ANALYZE has been executed on the tables or columns involved in the query to collect statistics.

Execute EXPLAIN VERBOSE to analyze the execution plan and check the warning information.

WARNING:Statistics in some tables or columns(public.lineitem(l_receiptdate,l_commitdate,l_orderkey, l_suppkey), public.orders(o_orderstatus,o_orderkey)) are not collected.
HINT:Do analyze for them in order to generate optimized plan.

To determine if poor query performance was caused by a lack of statistics in certain tables or columns, check if the following information exists in the log file located in the pg_log directory.

2017-06-14 17:28:30.336 CST 140644024579856 20971684 [BACKEND] LOG:Statistics in some tables or columns(public.lineitem(l_receiptdate, l_commitdate,l_orderkey,
.l_suppkey), public.orders(o_orderstatus,o_orderkey)) are not collected.
2017-06-14 17:28:30.336 CST 140644024579856 20971684 [BACKEND] HINT:Do analyze for them in order to generate optimized plan.

After confirming that ANALYZE has not been executed on the relevant tables or columns, you can execute ANALYZE on the tables or columns reported in the WARNING or logs to resolve the issue of slow query performance due to a lack of statistics

Example 2: Setting cost_param to Optimize Query Performance

For details, see Case: Configuring cost_param for Better Query Performance.

Example 3: Optimization is Not Accurate When Intermediate Results Exist in the Query Where JOIN Is Used for Multiple Tables

Symptom: Query the personnel who have checked in an Internet cafe within 15 minutes before and after the check-in of a specified person.

    
     
       
       
         SELECT
C.WBM,
C.DZQH,
C.DZ,
B.ZJHM,
B.SWKSSJ,
B.XWSJ
FROM
b_zyk_wbswxx A,
b_zyk_wbswxx B,
b_zyk_wbcs C
WHERE
A.ZJHM = '522522******3824'
AND A.WBDM = B.WBDM
AND A.WBDM = C.WBDM
AND abs(to_date(A.SWKSSJ,'yyyymmddHH24MISS') - to_date(B.SWKSSJ,'yyyymmddHH24MISS')) < INTERVAL '15 MINUTES'
ORDER BY
B.SWKSSJ,
B.ZJHM
limit 10 offset 0
;

        

      

    
   

Figure 1 shows the execution plan. This query takes about 12s.

Figure 1 Using an unlogged table (1)
Click to enlarge

Optimization analysis:

In the execution plan, index scan is used for node scanning, the Join Filter calculation in the external NEST LOOP IN statement consumes most of the query time, and the calculation uses the string addition and subtraction, and unequal-value comparison.

Use an unlogged table to record the Internet access time of the specified person. The start time and end time are processed during data insertion, and this reduces subsequent addition and subtraction operations.

      
       
         
         
           //Create a temporary unlogged table.
CREATE UNLOGGED TABLE temp_tsw
(
ZJHM         NVARCHAR2(18),
WBDM         NVARCHAR2(14),
SWKSSJ_START NVARCHAR2(14),
SWKSSJ_END   NVARCHAR2(14),
WBM          NVARCHAR2(70),
DZQH         NVARCHAR2(6),
DZ           NVARCHAR2(70),
IPDZ         NVARCHAR2(39)
)
;
//Insert the Internet access record of the specified person, and process the start time and end time.
INSERT INTO
temp_tsw
SELECT
A.ZJHM,
A.WBDM,
to_char((to_date(A.SWKSSJ,'yyyymmddHH24MISS') - INTERVAL '15 MINUTES'),'yyyymmddHH24MISS'),
to_char((to_date(A.SWKSSJ,'yyyymmddHH24MISS') + INTERVAL '15 MINUTES'),'yyyymmddHH24MISS'),
B.WBM,B.DZQH,B.DZ,B.IPDZ
FROM
b_zyk_wbswxx A,
b_zyk_wbcs B
WHERE
A.ZJHM='522522******3824' AND A.WBDM = B.WBDM
;

//Query the personnel who have check in an Internet cafe before and after 15 minutes of the check-in of the specified person. Convert their ID card number format to int8 in comparison.
SELECT
A.WBM,
A.DZQH,
A.DZ,
A.IPDZ,
B.ZJHM,
B.XM,
to_date(B.SWKSSJ,'yyyymmddHH24MISS') as SWKSSJ,
to_date(B.XWSJ,'yyyymmddHH24MISS') as XWSJ,
B.SWZDH
FROM temp_tsw A,
b_zyk_wbswxx B
WHERE
A.ZJHM <> B.ZJHM
AND A.WBDM = B.WBDM
AND (B.SWKSSJ)::int8 > (A.swkssj_start)::int8
AND (B.SWKSSJ)::int8 < (A.swkssj_end)::int8
order by
B.SWKSSJ,
B.ZJHM
limit 10 offset 0
;

          

        

      
     

The query takes about 7s. Figure 2 shows the execution plan.

Figure 2 Using an unlogged table (2)
Click to enlarge

In the previous plan, Hash Join has been executed, and a Hash table has been created for the large table b_zyk_wbswxx. The table contains large amounts of data, so the creation takes long time.
temp_tsw contains only hundreds of records, and an equal-value connection is created between temp_tsw and b_zyk_wbswxx using wbdm (the Internet cafe code). Therefore, if JOIN is changed to NEST LOOP JOIN, index scan can be used for node scanning, and the performance will be boosted.
Execute the following statement to change JOIN to NEST LOOP JOIN.
1

SET enable_hashjoin = off;
Figure 3 shows the execution plan. The query takes about 3s.

Figure 3 Using an unlogged table (3)

Save the query result set in the unlogged table for paging display.

If paging display needs to be achieved on the upper-layer application page, change the offset value to determine the result set on the target page. In this way, the previous query statement will be executed every time after a page turning operation, which causes long response latency.

To resolve this problem, you are advised to use the unlogged table to save the result set.

      
       
         
         
           //Create an unlogged table to save the result set.
CREATE UNLOGGED TABLE temp_result
(
WBM      NVARCHAR2(70),
DZQH     NVARCHAR2(6),
DZ       NVARCHAR2(70),
IPDZ     NVARCHAR2(39),
ZJHM     NVARCHAR2(18),
XM       NVARCHAR2(30),
SWKSSJ   date,
XWSJ     date,
SWZDH    NVARCHAR2(32)
);

//Insert the result set to the unlogged table. The insertion takes about 3s.
INSERT INTO
temp_result
SELECT
A.WBM,
A.DZQH,
A.DZ,
A.IPDZ,
B.ZJHM,
B.XM,
to_date(B.SWKSSJ,'yyyymmddHH24MISS') as SWKSSJ,
to_date(B.XWSJ,'yyyymmddHH24MISS') as XWSJ,
B.SWZDH
FROM temp_tsw A,
b_zyk_wbswxx B
WHERE
A.ZJHM <> B.ZJHM
AND A.WBDM = B.WBDM
AND (B.SWKSSJ)::int8 > (A.swkssj_start)::int8
AND (B.SWKSSJ)::int8 < (A.swkssj_end)::int8
;

//Perform paging query on the result set. The paging query takes about 10 ms.
SELECT
*
FROM
temp_result
ORDER BY
SWKSSJ,
ZJHM
LIMIT 10 OFFSET 0;

          

        

      
     